I have a setup that sort of gives what I'm looking for, but Emacs complains that I'm not doing it right.
I have a large number of org-mode appointments that depend on the date of Easter. They are currently in the form of diary sexp, but I don't really care what form they're in; what I'm really trying to find is a general solution to "When today is X days before Easter, show arbitrary message 'X' in the org agenda. When today is Y days after Easter, show arbitrary message 'Y' in the org agenda." (In all of my cases, Easter means "the Easter within the current calendar year".)
Emacs elegantly figures out the correct date for Easter, but Emacs also has a hard-coded list of Christian holidays that aren't useful to me. Instead of a new hard-coded list of holidays, I'm looking to make arbitrary and changeable use of the Easter calculation - compared to today - without showing any holidays at all.