anniversary

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.”

2. (especially)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.

   “We are celebrating our tenth anniversary today.”

3. (broadly)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.”

   “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.”

   “Jonathon proposed to his [Jess's] mom on their three-month anniversary of meeting and married her on their fourth.”