n 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.
n (qualifier|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).
n (context|more specifically) Such, where the event is a wedding.