ANNIVERSARY is properly the yearly return of any remarkable day and was anciently called a Year-day or Mind-day, that is, a Memorial-day. See ANNUAL DAY, etc.
On some extraordinary occasions, such days have been registered and annually observed in gratitude to founders and benefactors.
The word is formed from "annus" (year) and "verto" (to turn), in regard of its returning every year.
In this sense, we say Anniversary Days, Anniversary Feasts, Anniversary Prayers, etc.