holiday-christian-holidays

In Lisp buffer C-j:

((holiday-easter-etc) (holiday-fixed 12 25 "Christmas") (if calendar-christian-all-holidays-flag (append (holiday-fixed 1 6 "Epiphany") (holiday-julian 12 25 "Eastern Orthodox Christmas") (holiday-greek-orthodox-easter) (holiday-fixed 8 15 "Assumption") (holiday-advent 0 "Advent"))))

Easter Sunday is there:

holiday-easter-etc

How does Emacs calculate it?, and how can I can put 'Easter Monday' in my calendar.

Best wishes,

Enda

P.S. Easter Monday is the day after Easter Sunday.