What does anniversary mean?
The definition of anniversary
An`ni*ver”sa*ry, a.
Etym: [L. anniversarius; annus year + vertere, versum, to turn: cf. F. anniversaire.]

Returning with the year, at a stated time; annual; yearly; as, an anniversary feast. Anniversary day (R. C. Ch.). See Anniversary, n., 2.

— Anniversary week, that week in the year in which the annual meetings of religious and benevolent societies are held in Boston and New York. [Eastern U. S.]

An`ni*ver”sa*ry, n.; pl. Anniversaries.
Etym: [Cf. F. anniversaire.]

1 The annual return of the day on which any notable event took place, or is wont to be celebrated; as, the anniversary of the Declaration of Independence.
2 (R. C. Ch.)
The day on which Mass is said yearly for the soul of a deceased person; the commemoration of some sacred event, as the dedication of a church or the consecration of a pope.
3 The celebration which takes place on an anniversary day. Dryden.