Anniversary- the date on which an event took place in a previous year:
“the 50th anniversary of the start of World War II”
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It’s just a marker. My beef is with the idiots in the media that breathlessly announce, “...celebrating the one month anniversary of such and such.” Those idiots...(self censored remark)
When I worked for That Insurance Company, we talked about policies’ “monthlyversary” and “quarterversary.”