anniversary

Etymology

From Middle English anniversary, from Medieval Latin anniversāria (diēs), anniversārium, from anniversārius (“yearly”), from annus (“year”) + vertere (“to turn”).

noun

1. A day that is an exact number of years (to the day) since a given significant event occurred. Often preceded by an ordinal number indicating the number of years.
   Today is the fiftieth anniversary of the end of the war.
   1. (especially) Such a day that commemorates a wedding.
      We are celebrating our tenth anniversary today.
2. (loosely) A day subsequent in time to a given event by some significant period other than a year (especially as prefixed by the amount of time in question).
   The occasion was the six-month anniversary of the Aug. 8 coup that brought General Óscar Humberto Mejía Victores to power. 27 Feb 1984, “Never Mind the Tranquil Facade”, in Time
   In a thinly veiled threat to Saddam Hussein, President George Bush marked the six-month anniversary of September 11th by reiterating America's commitment to prevent rogue countries obtaining weapons of mass destruction. 14 Mar 2002, ‘Politics this Week’, The Economist
   Jonathon proposed to his [Jess's] mom on their three-month anniversary of meeting and married her on their fourth. 2006, DB Schrock, Soulmonger Dot Com, page 28