Obviously every year is annual. Every two years is biennial. Does the English language have a term for every 18 months?
Learn English – term to describe an event which happens every 18 months
datesrecurring-eventssingle-word-requeststime
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Best Answer
sesquiennial
I cannot find a dictionary definition of this other than Wiktionary, and 0 hits on Google NGram.
But there are some examples of usage here