Merriam-Webster Collegiate Dictionary - anniversary

 noun  (plural -ries)  Etymology: Middle English anniversarie, from Medieval Latin anniversarium, from Latin, neuter of anniversarius returning annually, from annus year + versus, past participle of vertere to turn — more at annual, worth  Date: 13th century  1. the annual recurrence of a date marking a notable event; broadly a date that follows such an event by a specified period of time measured in units other than years the 6-month ~ of the accident  2. the celebration of an ~