cal(1) on Linux
The text 3bb607b6b39b, as Debian 10 (Buster) shipped it.
displays a calendar and the date of Easter
Flags it lists
-
-1 -
-3 -
-A -
-B -
-C -
-H -
-J -
-M -
-N -
-S -
-b -
-d -
-e -
-h -
-j -
-m -
-o -
-p -
-s -
-w -
-y