/ ˌænɪˈvɜːsəri; NAmE -ˈvɜːrs-/ noun
( pl. -ies ) a date that is an exact number of years after the date of an important or special event :
on the anniversary of his wife's death
to celebrate your wedding anniversary
the theatre's 25th anniversary celebrations
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WORD ORIGIN
Middle English : from Latin anniversarius returning yearly, from annus year + versus turning.