Douglas Harper's Etymology Dictionary
heortology
"study of religious feasts and calendars," 1900, from Greek heorte "feast" + -ology. The immediate source of the English word is in French or German.
Wiktionary
heortology
n. The study of religious festivals.
Wikipedia
Heortology
Heortology is the study of religious festivals. It was originally only used in respect of Christian festivals, but it now covers all religions, in particular those of Ancient Greece.