anniversary

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The annually recurring date of a past event, as of personal or historical importance.
• noun A date that follows a certain event by a specified amount of time.
• noun An event at which an anniversary is celebrated.

from The Century Dictionary.

• Returning with the revolution of the year; annual; yearly: as, an anniversary feast.
• In the University of Cambridge, commemoration-day, an occasion upon which degrees are conferred since the disuse of the acts. See act, n., 5.
• noun The annually recurring date of some past event; more generally, a day set apart in each year for some commemorative observance; a day for the annual celebration of some notable event, public or private.
• noun In the Roman Catholic Church, the yearly commemoration of the day of a person's death, by a mass offered for his soul, or such commemoration of his death daily for a year.
• noun The act of celebrating a day on its annual recurrence; a yearly commemoration, or (rarely) something done or prepared for such commemoration.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Returning with the year, at a stated time; annual; yearly.
• adjective (R. C. Ch.) See Anniversary, n., 2.
• adjective [Eastern U. S.] that week in the year in which the annual meetings of religious and benevolent societies are held in Boston and New York.
• noun The annual return of the day on which any notable event took place, or is wont to be celebrated.
• noun (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.
• noun The celebration which takes place on an anniversary day.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Of a significant event, a day that is an exact number of years (to the day) since the event occurred. Often preceded by an ordinal number indicating the number of years that have elapsed since the event.
• noun loosely A day subsequent in time to a given event by some (especially significant) factor other than a year (especially as prefixed by the amount of time in question).
• noun more specifically Such, where the event is a wedding.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the date on which an event occurred in some previous year (or the celebration of it)