The Collaborative International Dictionary of English v.0.48:

Anniversary \An`ni*ver"sa*ry\, a. [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.
   [1913 Webster]

   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.]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Anniversary \An`ni*ver"sa*ry\, n.; pl. Anniversaries. [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.
      [1913 Webster]

   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.
      [1913 Webster]

   3. The celebration which takes place on an anniversary day.
      --Dryden.
      [1913 Webster]

WordNet (r) 3.0 (2006):

anniversary
    n 1: the date on which an event occurred in some previous year
         (or the celebration of it) [syn: anniversary, day of
         remembrance]