naming: localise liturgical day names in any language

Add internal/naming, which renders temporal day names and celebration
display names from the calendar engine's slugs in any language. English is
the built-in baseline; a language is data -- an embedded lang/<code>.ini
(pl shipped) and/or a user file at <config dir>/names/<code>.ini that
overrides it key by key. Names compose from a small vocabulary plus
per-language format templates, so word order and grammatical case can
differ (e.g. Polish genitive "3. Niedziela Okresu Zwykłego"); any string a
language omits falls back to English.

- Move day-name generation out of calendar (calendar.HumanizeSlug removed,
  calendar stays a pure engine) into naming.DayName; the English golden
  cases carry over verbatim.
- naming.CelebrationName unifies the three duplicated resolvers
  (cli/readings/calfeed): name.<lang> -> English -> Latin -> humanized slug.
  Saint names stay in the calendar data (name.<lang>), overridable via
  calendar layers.
- config: ui_language now accepts any code (lower-cased, not clamped to
  en/pl) so names/<code>.ini applies; UI chrome still resolves en/pl and
  falls back to English. Add NamesDir(); wire naming.SetUserDir (and the
  previously unwired bible.SetUserCorporaDir) in the TUI and web mains too,
  so external corpora and name files work across all three binaries.

13 files changed, 672 insertions, 240 deletions

diff --git a/cmd/lectio-ui/main.go b/cmd/lectio-ui/main.go
index b08188c..1b995a4 100644
--- a/cmd/lectio-ui/main.go
+++ b/cmd/lectio-ui/main.go
@@ -13,6 +13,7 @@ import (
 	"github.com/lukaszkasprzak/lectio/internal/bible"
 	"github.com/lukaszkasprzak/lectio/internal/bookmarks"
 	"github.com/lukaszkasprzak/lectio/internal/config"
+	"github.com/lukaszkasprzak/lectio/internal/naming"
 	"github.com/lukaszkasprzak/lectio/internal/tui"
 )
 
@@ -64,6 +65,12 @@ func run(args []string, stdout, stderr io.Writer) int {
 		fmt.Fprintln(stderr, "lectio-ui:", err)
 		return 1
 	}
+	if dir, err := config.CorporaDir(); err == nil {
+		bible.SetUserCorporaDir(dir)
+	}
+	if dir, err := config.NamesDir(); err == nil {
+		naming.SetUserDir(dir)
+	}
 
 	var all, offline, reader bool
 	var bibleVer, lectionary string
diff --git a/cmd/lectio-web/main.go b/cmd/lectio-web/main.go
index 327496a..683fabe 100644
--- a/cmd/lectio-web/main.go
+++ b/cmd/lectio-web/main.go
@@ -7,7 +7,9 @@ import (
 	"io"
 	"os"
 
+	"github.com/lukaszkasprzak/lectio/internal/bible"
 	"github.com/lukaszkasprzak/lectio/internal/config"
+	"github.com/lukaszkasprzak/lectio/internal/naming"
 	"github.com/lukaszkasprzak/lectio/internal/web"
 )
 
@@ -48,6 +50,12 @@ func run(args []string, stdout, stderr io.Writer) int {
 		fmt.Fprintln(stderr, "lectio-web:", err)
 		return 1
 	}
+	if dir, err := config.CorporaDir(); err == nil {
+		bible.SetUserCorporaDir(dir)
+	}
+	if dir, err := config.NamesDir(); err == nil {
+		naming.SetUserDir(dir)
+	}
 
 	var offline bool
 	var port int
diff --git a/internal/calendar/names.go b/internal/calendar/names.go
deleted file mode 100644
index 935b60c..0000000
--- a/internal/calendar/names.go
+++ /dev/null
@@ -1,151 +0,0 @@
-package calendar
-
-import (
-	"regexp"
-	"strconv"
-	"strings"
-)
-
-var weekdayName = map[string]string{
-	"mon": "Monday", "tue": "Tuesday", "wed": "Wednesday", "thu": "Thursday", "fri": "Friday", "sat": "Saturday", "sun": "Sunday",
-	"monday": "Monday", "tuesday": "Tuesday", "wednesday": "Wednesday", "thursday": "Thursday", "friday": "Friday", "saturday": "Saturday", "sunday": "Sunday",
-}
-
-// namedDay maps a computed temporal slug (with any "ef-" prefix removed) to its
-// proper English name, for days that have one.
-var namedDay = map[string]string{
-	"triduum-thu": "Holy Thursday", "triduum-fri": "Good Friday", "triduum-sat": "Holy Saturday",
-	"maundy-thursday": "Holy Thursday", "good-friday": "Good Friday", "holy-saturday": "Holy Saturday",
-	"palm-sunday": "Palm Sunday", "passion-sunday": "Passion Sunday", "low-sunday": "Second Sunday of Easter",
-	"trinity-sunday": "The Most Holy Trinity", "trinity": "Trinity Sunday",
-	"corpus-christi": "The Body and Blood of Christ", "sacred-heart": "The Most Sacred Heart of Jesus",
-	"christ-the-king": "Our Lord Jesus Christ, King of the Universe",
-	"easter-sunday":   "Easter Sunday", "pentecost": "Pentecost Sunday", "ascension": "The Ascension of the Lord",
-	"holy-family": "The Holy Family of Jesus, Mary and Joseph", "baptism-of-the-lord": "The Baptism of the Lord",
-	"epiphany": "The Epiphany of the Lord", "christmas": "The Nativity of the Lord", "nativity": "The Nativity of the Lord",
-	"circumcision": "The Circumcision of the Lord", "ash-wednesday": "Ash Wednesday",
-	"ascension-vigil": "Vigil of the Ascension", "pentecost-vigil": "Vigil of Pentecost",
-	"christmas-sunday-sun":                   "Second Sunday after the Nativity",
-	"mary-mother-of-god-octave-of-christmas": "Mary, the Holy Mother of God",
-}
-
-var (
-	reSundayNum = regexp.MustCompile(`^(ordinary|advent|lent|easter)-sunday-(\d+)$`)
-	reWeekday   = regexp.MustCompile(`^(.+)-(\d+)-([a-z]+)$`) // <season>-<week>-<weekday>
-	reOctave    = regexp.MustCompile(`^easter-octave-([a-z]+)$`)
-	reAfterAsh  = regexp.MustCompile(`^lent-after-ashes-([a-z]+)$`)
-	reHolyWeek  = regexp.MustCompile(`^holy-week-([a-z]+)$`)
-	reAdventDec = regexp.MustCompile(`^advent-dec-(\d+)$`)
-	reXmasDate  = regexp.MustCompile(`^christmas-(dec|jan)-(\d+)$`)
-	reAftEpiph  = regexp.MustCompile(`^christmas-after-epiphany-([a-z]+)$`)
-	reEmber     = regexp.MustCompile(`^(september|advent)-ember-([a-z]+)$`)
-	reSeasonSun = regexp.MustCompile(`^(.+)-sunday-(\d+)$`)
-)
-
-func ordinal(n int) string {
-	s := strconv.Itoa(n)
-	if n%100 >= 11 && n%100 <= 13 {
-		return s + "th"
-	}
-	switch n % 10 {
-	case 1:
-		return s + "st"
-	case 2:
-		return s + "nd"
-	case 3:
-		return s + "rd"
-	}
-	return s + "th"
-}
-
-func titleCase(s string) string {
-	small := map[string]bool{"of": true, "the": true, "in": true, "after": true, "before": true}
-	words := strings.Fields(s)
-	for i, w := range words {
-		if i > 0 && small[w] {
-			continue
-		}
-		words[i] = strings.ToUpper(w[:1]) + w[1:]
-	}
-	return strings.Join(words, " ")
-}
-
-// seasonPhrase renders a season slug ("ordinary", "time-after-pentecost") and the
-// preposition that introduces its numbered weeks.
-func seasonPhrase(season string) (name, prep string) {
-	switch season {
-	case "ordinary":
-		return "Ordinary Time", "in"
-	case "advent", "lent", "easter", "septuagesima", "passiontide":
-		return titleCase(season), "of"
-	}
-	if rest := strings.TrimPrefix(season, "time-after-"); rest != season {
-		return titleCase(rest), "after" // e.g. "Week after Pentecost"
-	}
-	return titleCase(strings.ReplaceAll(season, "-", " ")), "of"
-}
-
-// HumanizeSlug turns a computed temporal-day slug into a proper English name (used
-// when a temporal celebration carries no explicit name).
-func HumanizeSlug(slug string) string {
-	s := strings.TrimPrefix(slug, "ef-")
-	if n, ok := namedDay[s]; ok {
-		return n
-	}
-	if m := reSundayNum.FindStringSubmatch(s); m != nil {
-		n, _ := strconv.Atoi(m[2])
-		name, _ := seasonPhrase(m[1])
-		if m[1] == "ordinary" {
-			return ordinal(n) + " Sunday in " + name
-		}
-		return ordinal(n) + " Sunday of " + name
-	}
-	if m := reOctave.FindStringSubmatch(s); m != nil {
-		return weekdayName[m[1]] + " in the Octave of Easter"
-	}
-	if m := reAfterAsh.FindStringSubmatch(s); m != nil {
-		if m[1] == "wed" {
-			return "Ash Wednesday"
-		}
-		return weekdayName[m[1]] + " after Ash Wednesday"
-	}
-	if m := reHolyWeek.FindStringSubmatch(s); m != nil {
-		return weekdayName[m[1]] + " of Holy Week"
-	}
-	if m := reAdventDec.FindStringSubmatch(s); m != nil {
-		n, _ := strconv.Atoi(m[1])
-		return "December " + strconv.Itoa(n)
-	}
-	if m := reXmasDate.FindStringSubmatch(s); m != nil {
-		n, _ := strconv.Atoi(m[2])
-		if m[1] == "dec" {
-			return "December " + strconv.Itoa(n)
-		}
-		return "January " + strconv.Itoa(n)
-	}
-	if m := reAftEpiph.FindStringSubmatch(s); m != nil {
-		return weekdayName[m[1]] + " after the Epiphany"
-	}
-	if m := reEmber.FindStringSubmatch(s); m != nil {
-		return "Ember " + weekdayName[m[2]] + " of " + titleCase(m[1])
-	}
-	if m := reSeasonSun.FindStringSubmatch(s); m != nil { // <multiword-season>-sunday-N (EF)
-		n, _ := strconv.Atoi(m[2])
-		name, prep := seasonPhrase(m[1])
-		return ordinal(n) + " Sunday " + prep + " " + name
-	}
-	if m := reWeekday.FindStringSubmatch(s); m != nil {
-		if wd, ok := weekdayName[m[3]]; ok {
-			n, _ := strconv.Atoi(m[2])
-			name, prep := seasonPhrase(m[1])
-			if n == 0 { // the pre-Palm-Sunday week of Passiontide has no week number
-				if m[1] == "passiontide" {
-					return wd + " of Passion Week"
-				}
-				return wd + " " + prep + " " + name
-			}
-			return wd + " of the " + ordinal(n) + " Week " + prep + " " + name
-		}
-	}
-	return titleCase(strings.ReplaceAll(s, "-", " "))
-}
diff --git a/internal/calendar/names_test.go b/internal/calendar/names_test.go
deleted file mode 100644
index 217f481..0000000
--- a/internal/calendar/names_test.go
+++ /dev/null
@@ -1,27 +0,0 @@
-package calendar
-
-import "testing"
-
-func TestHumanizeSlug(t *testing.T) {
-	cases := map[string]string{
-		"ordinary-sunday-11":            "11th Sunday in Ordinary Time",
-		"ordinary-11-tue":               "Tuesday of the 11th Week in Ordinary Time",
-		"advent-2-mon":                  "Monday of the 2nd Week of Advent",
-		"advent-dec-17":                 "December 17",
-		"lent-after-ashes-wed":          "Ash Wednesday",
-		"lent-after-ashes-thu":          "Thursday after Ash Wednesday",
-		"triduum-fri":                   "Good Friday",
-		"easter-octave-mon":             "Monday in the Octave of Easter",
-		"christmas-jan-2":               "January 2",
-		"christmas-after-epiphany-mon":  "Monday after the Epiphany",
-		"trinity-sunday":                "The Most Holy Trinity",
-		"ef-time-after-pentecost-4-tue": "Tuesday of the 4th Week after Pentecost",
-		"ef-passiontide-0-thursday":     "Thursday of Passion Week",
-		"ef-september-ember-wed":        "Ember Wednesday of September",
-	}
-	for slug, want := range cases {
-		if got := HumanizeSlug(slug); got != want {
-			t.Errorf("HumanizeSlug(%q) = %q, want %q", slug, got, want)
-		}
-	}
-}
diff --git a/internal/calfeed/build.go b/internal/calfeed/build.go
index 216daff..8b4f7f0 100644
--- a/internal/calfeed/build.go
+++ b/internal/calfeed/build.go
@@ -4,6 +4,7 @@ import (
 	"time"
 
 	"github.com/lukaszkasprzak/lectio/internal/calendar"
+	"github.com/lukaszkasprzak/lectio/internal/naming"
 )
 
 // Build computes the day list from..to inclusive (calendar.Compute per date)
@@ -49,19 +50,12 @@ func celView(uiLang string, c calendar.Celebration) CelView {
 	}
 }
 
-// celebrationName resolves c's name in uiLang, falling back to English, then
-// a humanized slug (with the "ef-" prefix stripped and dashes turned to
-// spaces), then "(feria)" for an unnamed temporal day. Mirrors
-// cli.celebrationName.
+// celebrationName resolves c's name in uiLang via naming.CelebrationName
+// (name.<lang> -> English -> Latin -> humanized slug), then "(feria)" for an
+// unnamed temporal day.
 func celebrationName(uiLang string, c calendar.Celebration) string {
-	if n := c.Name[uiLang]; n != "" {
+	if n := naming.CelebrationName(uiLang, c); n != "" {
 		return n
 	}
-	if n := c.Name["en"]; n != "" {
-		return n
-	}
-	if c.Slug == "" {
-		return "(feria)"
-	}
-	return calendar.HumanizeSlug(c.Slug)
+	return "(feria)"
 }
diff --git a/internal/cli/cli.go b/internal/cli/cli.go
index c4f063d..1fc7ac0 100644
--- a/internal/cli/cli.go
+++ b/internal/cli/cli.go
@@ -19,6 +19,7 @@ import (
 	"github.com/lukaszkasprzak/lectio/internal/export"
 	"github.com/lukaszkasprzak/lectio/internal/i18n"
 	"github.com/lukaszkasprzak/lectio/internal/liturgy"
+	"github.com/lukaszkasprzak/lectio/internal/naming"
 	"github.com/lukaszkasprzak/lectio/internal/readings"
 	"github.com/lukaszkasprzak/lectio/internal/render"
 )
@@ -203,6 +204,9 @@ func Run(args []string, stdin io.Reader, stdout, stderr io.Writer) int {
 	if dir, err := config.CorporaDir(); err == nil {
 		bible.SetUserCorporaDir(dir)
 	}
+	if dir, err := config.NamesDir(); err == nil {
+		naming.SetUserDir(dir)
+	}
 	if lectionary != "" {
 		cfg.Lectionary = lectionary
 	}
diff --git a/internal/cli/liturgy.go b/internal/cli/liturgy.go
index 07a87bc..9d7f3b6 100644
--- a/internal/cli/liturgy.go
+++ b/internal/cli/liturgy.go
@@ -11,6 +11,7 @@ import (
 	"github.com/lukaszkasprzak/lectio/internal/calendar"
 	"github.com/lukaszkasprzak/lectio/internal/config"
 	"github.com/lukaszkasprzak/lectio/internal/i18n"
+	"github.com/lukaszkasprzak/lectio/internal/naming"
 	"github.com/lukaszkasprzak/lectio/internal/render"
 )
 
@@ -149,23 +150,14 @@ func printLiturgicalDay(out io.Writer, cfg config.Config, tbl *bible.BookTable,
 	}
 }
 
-// celebrationName is the celebration's name in the UI language (falling back to
-// English, then a humanized slug for temporal days that carry no proper name).
+// celebrationName is the celebration's name in the UI language, resolved by
+// naming.CelebrationName (name.<lang> -> English -> Latin -> humanized slug).
+// A bare, unnamed feria renders as "(feria)".
 func celebrationName(cfg config.Config, c calendar.Celebration) string {
-	lang := "en"
-	if cfg.UILanguage == "pl" {
-		lang = "pl"
-	}
-	if n := c.Name[lang]; n != "" {
-		return n
-	}
-	if n := c.Name["en"]; n != "" {
+	if n := naming.CelebrationName(cfg.UILanguage, c); n != "" {
 		return n
 	}
-	if c.Slug == "" {
-		return "(feria)"
-	}
-	return calendar.HumanizeSlug(c.Slug)
+	return "(feria)"
 }
 
 func rankLabel(r calendar.Rank) string {
diff --git a/internal/config/config.go b/internal/config/config.go
index 299c942..c9a00f2 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -37,7 +37,7 @@ const configHeader = `# lectio configuration (INI). Full-line comments only (# o
 #   width             integer; 0 = detect terminal width
 #   all               true | false             (all readings, or just the gospel)
 #   offline           true | false             (never fetch; cache/sigla only)
-#   ui_language       en | pl                  (interface chrome; readings stay source-language)
+#   ui_language       any code (en, pl, ...)   (chrome is en/pl, else English; day/saint names from names/<code>.ini)
 #   reading_lang      language for offline reading text (e.g. en, pl); blank follows ui_language
 #   reading_version   force a specific bible corpus code (e.g. drb, wuj); blank = auto-resolve
 #   sigla_style       auto | polish | english | latin  (citation dialect; auto follows ui_language)
@@ -131,21 +131,14 @@ func NormalizeDisplay(display string) string {
 	return d
 }
 
-// validUILanguages are the two UI chrome languages lectio understands.
-var validUILanguages = map[string]bool{
-	"en": true,
-	"pl": true,
-}
-
-// NormalizeUILanguage lower-cases lang and falls back to "en" when it is not
-// one of validUILanguages -- the same lenient style as NormalizeDisplay (no
-// error, just a safe default).
+// NormalizeUILanguage lower-cases and trims a ui_language code. Any code is
+// accepted: it selects day- and saint-name localisation (names/<code>.ini, via
+// internal/naming) and, for the UI chrome, resolves through i18n.Get, which
+// falls back to English for anything other than the built-in "en"/"pl". So a
+// user can set ui_language = fr, ship a names/fr.ini, and get French day names
+// with English chrome -- no rebuild.
 func NormalizeUILanguage(lang string) string {
-	l := strings.ToLower(lang)
-	if !validUILanguages[l] {
-		return "en"
-	}
-	return l
+	return strings.ToLower(strings.TrimSpace(lang))
 }
 
 // NormalizeSiglaStyle maps a sigla_style setting to one of "auto", "polish",
@@ -354,6 +347,19 @@ func CorporaDir() (string, error) {
 	return filepath.Join(filepath.Dir(p), "corpora"), nil
 }
 
+// NamesDir is the user directory for drop-in day-name language files
+// (<code>.ini), alongside calendars/ and corpora/ under the lectio config dir.
+// A file here overrides the built-in day names for its language key by key;
+// combined with ui_language = <code>, it localises the daily view to any
+// language without a rebuild. Mirrors CalendarsDir/CorporaDir.
+func NamesDir() (string, error) {
+	p, _, err := configPath()
+	if err != nil {
+		return "", err
+	}
+	return filepath.Join(filepath.Dir(p), "names"), nil
+}
+
 // BooksPath returns the path to the optional user books.ini (in the same
 // directory as the config file). There is no embedded seed on disk -- absence
 // means "use the built-in defaults".
diff --git a/internal/config/config_test.go b/internal/config/config_test.go
index 1035426..dd75808 100644
--- a/internal/config/config_test.go
+++ b/internal/config/config_test.go
@@ -169,18 +169,21 @@ func TestUILanguageLoadsPL(t *testing.T) {
 	}
 }
 
-func TestUILanguageNormalizesUnknown(t *testing.T) {
+func TestUILanguagePreservesAnyCode(t *testing.T) {
+	// Any ui_language code is kept (lower-cased): it drives day/saint-name
+	// localisation via names/<code>.ini. The UI chrome, resolved through
+	// i18n.Get, still falls back to English for a code it has no table for.
 	dir := t.TempDir()
 	t.Setenv("XDG_CONFIG_HOME", dir)
 	os.MkdirAll(filepath.Join(dir, "lectio"), 0o755)
 	os.WriteFile(filepath.Join(dir, "lectio", "config.toml"),
-		[]byte("ui_language = \"bogus\"\n"), 0o644)
+		[]byte("ui_language = \"FR\"\n"), 0o644)
 	cfg, err := Load()
 	if err != nil {
 		t.Fatal(err)
 	}
-	if cfg.UILanguage != "en" {
-		t.Errorf("UILanguage = %q, want %q (normalized from bogus)", cfg.UILanguage, "en")
+	if cfg.UILanguage != "fr" {
+		t.Errorf("UILanguage = %q, want %q (lower-cased, not clamped)", cfg.UILanguage, "fr")
 	}
 }
 
diff --git a/internal/naming/lang/pl.ini b/internal/naming/lang/pl.ini
new file mode 100644
index 0000000..f7a3217
--- /dev/null
+++ b/internal/naming/lang/pl.ini
@@ -0,0 +1,101 @@
+# Polish (pl) temporal day names for lectio.
+#
+# This file overrides the built-in English baseline key by key: anything left
+# out falls back to English. Copy it to <config dir>/names/pl.ini to customise,
+# or use it as a template for another language (<config dir>/names/<code>.ini,
+# then set ui_language = <code>). Placeholders in [templates]:
+#   {ord}  ordinal (see [ordinal])     {wd}    weekday (see [weekdays])
+#   {season} season name (see below)   {prep}  preposition (see [season_prep])
+#   {day}  day-of-month number         {month} month name (see [months])
+# Season names are given in the genitive case Polish uses after "Niedziela"/
+# "tygodnia", so no linking preposition is needed ([season_prep] left empty).
+
+[templates]
+sunday          = {ord} Niedziela {season}
+weekday         = {wd} {ord} tygodnia {season}
+weekday_no_week = {wd} {season}
+octave_easter   = {wd} w Oktawie Wielkanocnej
+after_ashes     = {wd} po Środzie Popielcowej
+holy_week       = {wd} Wielkiego Tygodnia
+after_epiphany  = {wd} po Objawieniu Pańskim
+ember           = {wd} suchych dni {season}
+passion_week    = {wd} Tygodnia Męki Pańskiej
+date            = {day} {month}
+
+[ordinal]
+format = %d.
+
+[weekdays]
+mon = Poniedziałek
+tue = Wtorek
+wed = Środa
+thu = Czwartek
+fri = Piątek
+sat = Sobota
+sun = Niedziela
+
+[months]
+1  = stycznia
+2  = lutego
+3  = marca
+4  = kwietnia
+5  = maja
+6  = czerwca
+7  = lipca
+8  = sierpnia
+9  = września
+10 = października
+11 = listopada
+12 = grudnia
+
+[seasons]
+ordinary             = Okresu Zwykłego
+advent               = Adwentu
+lent                 = Wielkiego Postu
+easter               = Okresu Wielkanocnego
+septuagesima         = Przedpościa
+passiontide          = Męki Pańskiej
+time-after-pentecost = po Zesłaniu Ducha Świętego
+time-after-epiphany  = po Objawieniu Pańskim
+september            = września
+
+[season_prep]
+ordinary             =
+advent               =
+lent                 =
+easter               =
+septuagesima         =
+passiontide          =
+time-after-pentecost =
+time-after-epiphany  =
+september            =
+
+[named]
+triduum-thu     = Wielki Czwartek
+triduum-fri     = Wielki Piątek
+triduum-sat     = Wielka Sobota
+maundy-thursday = Wielki Czwartek
+good-friday     = Wielki Piątek
+holy-saturday   = Wielka Sobota
+palm-sunday     = Niedziela Palmowa
+passion-sunday  = Niedziela Męki Pańskiej
+low-sunday      = 2. Niedziela Wielkanocna
+trinity-sunday  = Uroczystość Najświętszej Trójcy
+trinity         = Niedziela Trójcy Świętej
+corpus-christi  = Najświętszego Ciała i Krwi Chrystusa
+sacred-heart    = Najświętszego Serca Pana Jezusa
+christ-the-king = Jezusa Chrystusa Króla Wszechświata
+easter-sunday   = Niedziela Zmartwychwstania Pańskiego
+pentecost       = Zesłanie Ducha Świętego
+ascension       = Wniebowstąpienie Pańskie
+holy-family     = Świętej Rodziny: Jezusa, Maryi i Józefa
+baptism-of-the-lord = Chrzest Pański
+epiphany        = Objawienie Pańskie
+christmas       = Narodzenie Pańskie
+nativity        = Narodzenie Pańskie
+circumcision    = Obrzezanie Pańskie
+ash-wednesday   = Środa Popielcowa
+ascension-vigil = Wigilia Wniebowstąpienia
+pentecost-vigil = Wigilia Zesłania Ducha Świętego
+christmas-sunday-sun = 2. Niedziela po Narodzeniu Pańskim
+mary-mother-of-god-octave-of-christmas = Świętej Bożej Rodzicielki Maryi
diff --git a/internal/naming/naming.go b/internal/naming/naming.go
new file mode 100644
index 0000000..ca47a15
--- /dev/null
+++ b/internal/naming/naming.go
@@ -0,0 +1,453 @@
+// Package naming renders human-readable liturgical day names from the calendar
+// engine's computed slugs, in any language. English is the built-in baseline
+// (builtinEN, below); every other language is data: an embedded lang/<code>.ini
+// shipped with lectio, and/or a user file at <names dir>/<code>.ini that
+// overrides it key by key. A partial translation never breaks -- any string a
+// language omits falls back to English.
+//
+// Two things are localised here: the generated TEMPORAL day names (e.g. "3rd
+// Sunday in Ordinary Time"), composed from a small vocabulary plus per-language
+// format templates so word order and grammatical case can differ; and the
+// display name of any celebration (CelebrationName), which prefers the
+// calendar entry's own name.<lang> and falls back through English, Latin, and
+// finally the humanized slug. Saint names themselves live in the calendar data
+// (name.<lang>), not here.
+package naming
+
+import (
+	"embed"
+	"fmt"
+	"os"
+	"path/filepath"
+	"regexp"
+	"strconv"
+	"strings"
+	"sync"
+
+	"github.com/lukaszkasprzak/lectio/internal/calendar"
+	"github.com/lukaszkasprzak/lectio/internal/ini"
+)
+
+//go:embed lang
+var langFS embed.FS
+
+// Table is one language's temporal-naming data. Every field is optional in a
+// language file: a missing entry falls back to the English baseline (for maps,
+// per key), so a translator fills in only what they want to change.
+type Table struct {
+	Weekdays   map[string]string // mon,tue,wed,thu,fri,sat,sun -> weekday name
+	NamedDays  map[string]string // temporal slug -> proper name (Good Friday, ...)
+	Seasons    map[string]string // season slug -> localised season name
+	SeasonPrep map[string]string // season slug -> preposition joining it to a week/Sunday
+	Months     []string          // 12 month names (index 0 = January), for dated ferias
+	Templates  map[string]string // template id -> format with {ord}{wd}{season}{prep}{day}{month}
+	Ordinals   map[string]string // "1".."40" -> localised ordinal (optional; overrides OrdinalFmt)
+	OrdinalFmt string            // printf-style fallback ordinal, e.g. "%d." (optional)
+}
+
+// ---- temporal slug patterns (the calendar engine's own slug shapes) ---------
+
+var (
+	reSeasonSun = regexp.MustCompile(`^(.+)-sunday-(\d+)$`)                 // <season>-sunday-N (all numbered Sundays)
+	reOctave    = regexp.MustCompile(`^easter-octave-([a-z]+)$`)            // Octave of Easter weekday
+	reAfterAsh  = regexp.MustCompile(`^lent-after-ashes-([a-z]+)$`)         // Thu/Fri/Sat after Ash Wednesday
+	reHolyWeek  = regexp.MustCompile(`^holy-week-([a-z]+)$`)                // Mon-Wed of Holy Week
+	reAdventDec = regexp.MustCompile(`^advent-dec-(\d+)$`)                  // late-Advent dated feria
+	reXmasDate  = regexp.MustCompile(`^christmas-(dec|jan)-(\d+)$`)         // Christmastide dated feria
+	reAftEpiph  = regexp.MustCompile(`^christmas-after-epiphany-([a-z]+)$`) // weekday after Epiphany
+	reEmber     = regexp.MustCompile(`^(september|advent)-ember-([a-z]+)$`) // Ember day
+	reWeekday   = regexp.MustCompile(`^(.+)-(\d+)-([a-z]+)$`)               // <season>-<week>-<weekday>
+	rePlacehold = regexp.MustCompile(`\{[a-z]+\}`)                          // leftover {token} cleanup
+)
+
+// weekdayAbbrev maps any weekday token the slugs use (abbreviated or full) to
+// the canonical three-letter key the Table's Weekdays map is keyed by.
+var weekdayAbbrev = map[string]string{
+	"mon": "mon", "tue": "tue", "wed": "wed", "thu": "thu", "fri": "fri", "sat": "sat", "sun": "sun",
+	"monday": "mon", "tuesday": "tue", "wednesday": "wed", "thursday": "thu",
+	"friday": "fri", "saturday": "sat", "sunday": "sun",
+}
+
+// builtinEN is the English baseline every other language overlays. The season
+// name/preposition and ordinal for English are computed (enSeasonName,
+// enSeasonPrep, enOrdinal) rather than tabulated, so unlisted seasons still
+// read correctly; a language file supplies Seasons/SeasonPrep/Ordinals to
+// override them.
+var builtinEN = Table{
+	Weekdays: map[string]string{
+		"mon": "Monday", "tue": "Tuesday", "wed": "Wednesday", "thu": "Thursday",
+		"fri": "Friday", "sat": "Saturday", "sun": "Sunday",
+	},
+	NamedDays: map[string]string{
+		"triduum-thu": "Holy Thursday", "triduum-fri": "Good Friday", "triduum-sat": "Holy Saturday",
+		"maundy-thursday": "Holy Thursday", "good-friday": "Good Friday", "holy-saturday": "Holy Saturday",
+		"palm-sunday": "Palm Sunday", "passion-sunday": "Passion Sunday", "low-sunday": "Second Sunday of Easter",
+		"trinity-sunday": "The Most Holy Trinity", "trinity": "Trinity Sunday",
+		"corpus-christi": "The Body and Blood of Christ", "sacred-heart": "The Most Sacred Heart of Jesus",
+		"christ-the-king": "Our Lord Jesus Christ, King of the Universe",
+		"easter-sunday":   "Easter Sunday", "pentecost": "Pentecost Sunday", "ascension": "The Ascension of the Lord",
+		"holy-family": "The Holy Family of Jesus, Mary and Joseph", "baptism-of-the-lord": "The Baptism of the Lord",
+		"epiphany": "The Epiphany of the Lord", "christmas": "The Nativity of the Lord", "nativity": "The Nativity of the Lord",
+		"circumcision": "The Circumcision of the Lord", "ash-wednesday": "Ash Wednesday",
+		"ascension-vigil": "Vigil of the Ascension", "pentecost-vigil": "Vigil of Pentecost",
+		"christmas-sunday-sun":                   "Second Sunday after the Nativity",
+		"mary-mother-of-god-octave-of-christmas": "Mary, the Holy Mother of God",
+	},
+	Months: []string{"January", "February", "March", "April", "May", "June",
+		"July", "August", "September", "October", "November", "December"},
+	Templates: map[string]string{
+		"sunday":          "{ord} Sunday {prep} {season}",
+		"weekday":         "{wd} of the {ord} Week {prep} {season}",
+		"weekday_no_week": "{wd} {prep} {season}",
+		"octave_easter":   "{wd} in the Octave of Easter",
+		"after_ashes":     "{wd} after Ash Wednesday",
+		"holy_week":       "{wd} of Holy Week",
+		"after_epiphany":  "{wd} after the Epiphany",
+		"ember":           "Ember {wd} of {season}",
+		"passion_week":    "{wd} of Passion Week",
+		"date":            "{month} {day}",
+	},
+	Seasons:    map[string]string{},
+	SeasonPrep: map[string]string{},
+	Ordinals:   map[string]string{},
+}
+
+// ---- table resolution + caching ---------------------------------------------
+
+var (
+	tblMu    sync.Mutex
+	tblCache = map[string]Table{}
+	userDir  string
+)
+
+// SetUserDir points naming at the drop-in dir holding user language files
+// (<dir>/<code>.ini) and invalidates the cache. Empty string disables it.
+func SetUserDir(dir string) {
+	tblMu.Lock()
+	userDir = dir
+	tblCache = map[string]Table{}
+	tblMu.Unlock()
+}
+
+// tableFor returns the merged table for lang: the English baseline, overlaid by
+// the embedded lang/<code>.ini (if lectio ships one), overlaid by the user's
+// <dir>/<code>.ini (if present).
+func tableFor(lang string) Table {
+	tblMu.Lock()
+	defer tblMu.Unlock()
+	if t, ok := tblCache[lang]; ok {
+		return t
+	}
+	t := builtinEN.clone()
+	if lang != "" && lang != "en" {
+		if data, err := langFS.ReadFile("lang/" + lang + ".ini"); err == nil {
+			t.overlay(parseTable(data))
+		}
+	}
+	if userDir != "" && lang != "" {
+		if data, err := os.ReadFile(filepath.Join(userDir, lang+".ini")); err == nil {
+			t.overlay(parseTable(data))
+		}
+	}
+	tblCache[lang] = t
+	return t
+}
+
+func (t Table) clone() Table {
+	c := Table{
+		Weekdays: cloneMap(t.Weekdays), NamedDays: cloneMap(t.NamedDays),
+		Seasons: cloneMap(t.Seasons), SeasonPrep: cloneMap(t.SeasonPrep),
+		Templates: cloneMap(t.Templates), Ordinals: cloneMap(t.Ordinals),
+		OrdinalFmt: t.OrdinalFmt,
+	}
+	c.Months = append([]string(nil), t.Months...)
+	return c
+}
+
+// overlay copies o's non-empty entries onto t (per key), so a partial language
+// file overrides only the strings it provides.
+func (t *Table) overlay(o Table) {
+	mergeMap(t.Weekdays, o.Weekdays)
+	mergeMap(t.NamedDays, o.NamedDays)
+	mergeMap(t.Seasons, o.Seasons)
+	mergeMap(t.SeasonPrep, o.SeasonPrep)
+	mergeMap(t.Templates, o.Templates)
+	mergeMap(t.Ordinals, o.Ordinals)
+	if o.OrdinalFmt != "" {
+		t.OrdinalFmt = o.OrdinalFmt
+	}
+	for i, m := range o.Months {
+		if m != "" && i < len(t.Months) {
+			t.Months[i] = m
+		}
+	}
+}
+
+func cloneMap(m map[string]string) map[string]string {
+	c := make(map[string]string, len(m))
+	for k, v := range m {
+		c[k] = v
+	}
+	return c
+}
+
+// mergeMap copies every entry of src over dst, including empty values: an empty
+// override is respected where it is meaningful (a season_prep a language leaves
+// blank because it joins by grammatical case) and harmless elsewhere, since the
+// getters fall back to English when a looked-up string is empty.
+func mergeMap(dst, src map[string]string) {
+	for k, v := range src {
+		dst[k] = v
+	}
+}
+
+// ---- public API -------------------------------------------------------------
+
+// DayName renders a temporal day slug as a proper name in lang, e.g.
+// DayName("ordinary-sunday-11", "en") == "11th Sunday in Ordinary Time".
+func DayName(slug, lang string) string {
+	return tableFor(lang).day(slug)
+}
+
+// CelebrationName is a celebration's display name in lang: its own name.<lang>
+// if present, else name.en, else name.la, else the humanized temporal slug.
+// Returns "" only for an unnamed celebration with an empty slug (the caller
+// decides how to render a bare feria).
+func CelebrationName(lang string, c calendar.Celebration) string {
+	if lang != "" {
+		if n := c.Name[lang]; n != "" {
+			return n
+		}
+	}
+	if n := c.Name["en"]; n != "" {
+		return n
+	}
+	if n := c.Name["la"]; n != "" {
+		return n
+	}
+	if c.Slug == "" {
+		return ""
+	}
+	return DayName(c.Slug, lang)
+}
+
+// ---- composition ------------------------------------------------------------
+
+func (t Table) day(slug string) string {
+	s := strings.TrimPrefix(slug, "ef-")
+	if n, ok := t.NamedDays[s]; ok {
+		return n
+	}
+	if m := reSeasonSun.FindStringSubmatch(s); m != nil {
+		return t.render("sunday", map[string]string{
+			"ord": t.ordinal(atoi(m[2])), "prep": t.prep(m[1]), "season": t.season(m[1]),
+		})
+	}
+	if m := reOctave.FindStringSubmatch(s); m != nil {
+		return t.render("octave_easter", map[string]string{"wd": t.weekday(m[1])})
+	}
+	if m := reAfterAsh.FindStringSubmatch(s); m != nil {
+		if weekdayAbbrev[m[1]] == "wed" {
+			return t.NamedDays["ash-wednesday"] // the day the ferias hang off of
+		}
+		return t.render("after_ashes", map[string]string{"wd": t.weekday(m[1])})
+	}
+	if m := reHolyWeek.FindStringSubmatch(s); m != nil {
+		return t.render("holy_week", map[string]string{"wd": t.weekday(m[1])})
+	}
+	if m := reAdventDec.FindStringSubmatch(s); m != nil {
+		return t.render("date", map[string]string{"month": t.month(12), "day": m[1]})
+	}
+	if m := reXmasDate.FindStringSubmatch(s); m != nil {
+		mon := 12
+		if m[1] == "jan" {
+			mon = 1
+		}
+		return t.render("date", map[string]string{"month": t.month(mon), "day": m[2]})
+	}
+	if m := reAftEpiph.FindStringSubmatch(s); m != nil {
+		return t.render("after_epiphany", map[string]string{"wd": t.weekday(m[1])})
+	}
+	if m := reEmber.FindStringSubmatch(s); m != nil {
+		return t.render("ember", map[string]string{"wd": t.weekday(m[2]), "season": t.season(m[1])})
+	}
+	if m := reWeekday.FindStringSubmatch(s); m != nil {
+		if _, ok := weekdayAbbrev[m[3]]; ok {
+			n := atoi(m[2])
+			if n == 0 { // Passiontide's pre-Palm-Sunday week carries no number
+				if m[1] == "passiontide" {
+					return t.render("passion_week", map[string]string{"wd": t.weekday(m[3])})
+				}
+				return t.render("weekday_no_week", map[string]string{
+					"wd": t.weekday(m[3]), "prep": t.prep(m[1]), "season": t.season(m[1]),
+				})
+			}
+			return t.render("weekday", map[string]string{
+				"wd": t.weekday(m[3]), "ord": t.ordinal(n), "prep": t.prep(m[1]), "season": t.season(m[1]),
+			})
+		}
+	}
+	return titleCase(strings.ReplaceAll(s, "-", " "))
+}
+
+// render fills a template's {tokens}, drops any left unfilled, and collapses
+// whitespace -- so a language with an empty preposition (case-joined seasons)
+// never leaves a double space.
+func (t Table) render(id string, vars map[string]string) string {
+	f := t.Templates[id]
+	if f == "" {
+		f = builtinEN.Templates[id]
+	}
+	for k, v := range vars {
+		f = strings.ReplaceAll(f, "{"+k+"}", v)
+	}
+	f = rePlacehold.ReplaceAllString(f, "")
+	return strings.Join(strings.Fields(f), " ")
+}
+
+func (t Table) weekday(tok string) string {
+	if ab, ok := weekdayAbbrev[tok]; ok {
+		if name := t.Weekdays[ab]; name != "" {
+			return name
+		}
+	}
+	return titleCase(tok)
+}
+
+func (t Table) month(n int) string {
+	if n >= 1 && n <= len(t.Months) && t.Months[n-1] != "" {
+		return t.Months[n-1]
+	}
+	return builtinEN.Months[n-1]
+}
+
+func (t Table) ordinal(n int) string {
+	if s, ok := t.Ordinals[strconv.Itoa(n)]; ok {
+		return s
+	}
+	if t.OrdinalFmt != "" {
+		return fmt.Sprintf(t.OrdinalFmt, n)
+	}
+	return enOrdinal(n)
+}
+
+func (t Table) season(slug string) string {
+	if s, ok := t.Seasons[slug]; ok {
+		return s
+	}
+	return enSeasonName(slug)
+}
+
+func (t Table) prep(slug string) string {
+	if p, ok := t.SeasonPrep[slug]; ok {
+		return p // may be intentionally empty
+	}
+	return enSeasonPrep(slug)
+}
+
+// ---- English fallbacks (match the historical HumanizeSlug behaviour) ---------
+
+func enSeasonName(slug string) string {
+	if slug == "ordinary" {
+		return "Ordinary Time"
+	}
+	if rest := strings.TrimPrefix(slug, "time-after-"); rest != slug {
+		return titleCase(rest)
+	}
+	return titleCase(strings.ReplaceAll(slug, "-", " "))
+}
+
+func enSeasonPrep(slug string) string {
+	if slug == "ordinary" {
+		return "in"
+	}
+	if strings.HasPrefix(slug, "time-after-") {
+		return "after"
+	}
+	return "of"
+}
+
+func enOrdinal(n int) string {
+	s := strconv.Itoa(n)
+	if n%100 >= 11 && n%100 <= 13 {
+		return s + "th"
+	}
+	switch n % 10 {
+	case 1:
+		return s + "st"
+	case 2:
+		return s + "nd"
+	case 3:
+		return s + "rd"
+	}
+	return s + "th"
+}
+
+func titleCase(s string) string {
+	small := map[string]bool{"of": true, "the": true, "in": true, "after": true, "before": true}
+	words := strings.Fields(s)
+	for i, w := range words {
+		if i > 0 && small[w] {
+			continue
+		}
+		if w != "" {
+			words[i] = strings.ToUpper(w[:1]) + w[1:]
+		}
+	}
+	return strings.Join(words, " ")
+}
+
+func atoi(s string) int { n, _ := strconv.Atoi(s); return n }
+
+// ---- language-file parsing --------------------------------------------------
+
+// parseTable reads a language INI into a (partial) Table. Sections:
+//
+//	[templates]   id = format
+//	[weekdays]    mon..sun = name
+//	[months]      1..12 = name
+//	[seasons]     <season slug> = name
+//	[season_prep] <season slug> = preposition (may be empty)
+//	[named]       <temporal slug> = name
+//	[ordinal]     format = %d.   |   1 = 1st, 2 = 2nd, ...
+func parseTable(data []byte) Table {
+	t := Table{
+		Weekdays: map[string]string{}, NamedDays: map[string]string{},
+		Seasons: map[string]string{}, SeasonPrep: map[string]string{},
+		Templates: map[string]string{}, Ordinals: map[string]string{},
+		Months: make([]string, 12),
+	}
+	secs, err := ini.Parse(data)
+	if err != nil {
+		return t
+	}
+	for _, s := range secs {
+		for _, p := range s.Pairs {
+			switch s.Name {
+			case "templates":
+				t.Templates[p.Key] = p.Val
+			case "weekdays":
+				if ab, ok := weekdayAbbrev[p.Key]; ok {
+					t.Weekdays[ab] = p.Val
+				}
+			case "months":
+				if i := atoi(p.Key); i >= 1 && i <= 12 {
+					t.Months[i-1] = p.Val
+				}
+			case "seasons":
+				t.Seasons[p.Key] = p.Val
+			case "season_prep":
+				t.SeasonPrep[p.Key] = p.Val
+			case "named":
+				t.NamedDays[p.Key] = p.Val
+			case "ordinal":
+				if p.Key == "format" {
+					t.OrdinalFmt = p.Val
+				} else {
+					t.Ordinals[p.Key] = p.Val
+				}
+			}
+		}
+	}
+	return t
+}
diff --git a/internal/naming/naming_test.go b/internal/naming/naming_test.go
new file mode 100644
index 0000000..a3bc49f
--- /dev/null
+++ b/internal/naming/naming_test.go
@@ -0,0 +1,55 @@
+package naming
+
+import "testing"
+
+// English must reproduce exactly what calendar.HumanizeSlug produced before the
+// name generation moved here (the golden cases carried over verbatim).
+func TestDayNameEnglish(t *testing.T) {
+	cases := map[string]string{
+		"ordinary-sunday-11":            "11th Sunday in Ordinary Time",
+		"ordinary-11-tue":               "Tuesday of the 11th Week in Ordinary Time",
+		"advent-2-mon":                  "Monday of the 2nd Week of Advent",
+		"advent-dec-17":                 "December 17",
+		"lent-after-ashes-wed":          "Ash Wednesday",
+		"lent-after-ashes-thu":          "Thursday after Ash Wednesday",
+		"triduum-fri":                   "Good Friday",
+		"easter-octave-mon":             "Monday in the Octave of Easter",
+		"christmas-jan-2":               "January 2",
+		"christmas-after-epiphany-mon":  "Monday after the Epiphany",
+		"trinity-sunday":                "The Most Holy Trinity",
+		"ef-time-after-pentecost-4-tue": "Tuesday of the 4th Week after Pentecost",
+		"ef-passiontide-0-thursday":     "Thursday of Passion Week",
+		"ef-september-ember-wed":        "Ember Wednesday of September",
+	}
+	for slug, want := range cases {
+		if got := DayName(slug, "en"); got != want {
+			t.Errorf("DayName(%q, en) = %q, want %q", slug, got, want)
+		}
+	}
+}
+
+// Polish must localise both named days and composed names, using its own word
+// order and genitive-case season names.
+func TestDayNamePolish(t *testing.T) {
+	cases := map[string]string{
+		"ordinary-sunday-11":            "11. Niedziela Okresu Zwykłego",
+		"ordinary-11-tue":               "Wtorek 11. tygodnia Okresu Zwykłego",
+		"advent-2-mon":                  "Poniedziałek 2. tygodnia Adwentu",
+		"advent-dec-17":                 "17 grudnia",
+		"triduum-fri":                   "Wielki Piątek",
+		"lent-after-ashes-wed":          "Środa Popielcowa",
+		"ef-time-after-pentecost-4-tue": "Wtorek 4. tygodnia po Zesłaniu Ducha Świętego",
+	}
+	for slug, want := range cases {
+		if got := DayName(slug, "pl"); got != want {
+			t.Errorf("DayName(%q, pl) = %q, want %q", slug, got, want)
+		}
+	}
+}
+
+// An unknown language falls back entirely to English.
+func TestDayNameUnknownLangFallsBackToEnglish(t *testing.T) {
+	if got := DayName("ordinary-sunday-3", "xx"); got != "3rd Sunday in Ordinary Time" {
+		t.Errorf("unknown lang = %q", got)
+	}
+}
diff --git a/internal/readings/offline.go b/internal/readings/offline.go
index 566fc7a..d0b8942 100644
--- a/internal/readings/offline.go
+++ b/internal/readings/offline.go
@@ -9,6 +9,7 @@ import (
 	"github.com/lukaszkasprzak/lectio/internal/calendar"
 	"github.com/lukaszkasprzak/lectio/internal/config"
 	"github.com/lukaszkasprzak/lectio/internal/liturgy"
+	"github.com/lukaszkasprzak/lectio/internal/naming"
 )
 
 // offlineLoad resolves a day's readings entirely from the embedded calendar
@@ -119,22 +120,8 @@ func dayInfo(cfg config.Config, day calendar.LiturgicalDay) liturgy.DayInfo {
 }
 
 // celebrationName is the observed celebration's name in the UI language,
-// falling back to English then a humanized slug for temporal days that carry no
-// proper name (mirrors the CLI's cli.celebrationName). An empty result omits
-// the header line.
+// resolved by naming.CelebrationName (name.<lang> -> English -> Latin ->
+// humanized slug). An empty result (unnamed feria) omits the header line.
 func celebrationName(cfg config.Config, c calendar.Celebration) string {
-	lang := "en"
-	if cfg.UILanguage == "pl" {
-		lang = "pl"
-	}
-	if n := c.Name[lang]; n != "" {
-		return n
-	}
-	if n := c.Name["en"]; n != "" {
-		return n
-	}
-	if c.Slug == "" {
-		return ""
-	}
-	return calendar.HumanizeSlug(c.Slug)
+	return naming.CelebrationName(cfg.UILanguage, c)
 }