Easter and its Easter-relative movable feasts for a year:

  $ colitur easter 2026
  easter 2026-04-05
  ash-wednesday 2026-02-18
  palm-sunday 2026-03-29
  ascension 2026-05-14
  pentecost 2026-05-24
  corpus-christi 2026-06-04

A year outside the supported domain is rejected (exit 2):

  $ colitur easter 1000
  colitur: year 1000 out of range 1583..9999
  [2]

No/garbage arguments give a usage error (exit 2):

  $ colitur
  colitur: usage: colitur easter <year>
  [2]