the date on which an event took place in a previous year 周年纪念日 the 50th anniversary of the Battle of Britain. 不列颠之战50周年纪念日。 ■the date on which a country or other institution was founded in a previous year 建国周年纪念日;成立周年纪念日 the 75th anniversary of the RAF. 皇家空军成立75周年庆。 ■the date on which a couple were married in a previous year 结婚纪念日 he even forgot our tenth anniversary!. 他甚至忘了我们结婚十周年纪念的日子!。
语源
Middle English : from Latin anniversarius 'returning yearly', from annus 'year' + versus 'turning'