Is there a term to describe an event which happens every 18 months?

sesquiennial

ocurring every year and a half.

Etymology: semi ("half") + que ("and")

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

FAME now has responsibility for the sesquiennial (every 18 months) Music Festivals which attract players from Europe, American continent

I think JoseK's answer is the correct one, but there are also a few google hits for 'semi-triannual' and even a few for the (more correct, because unambiguous) 'semi-triennial.' Either of these might be easier for casual readers to decipher than 'sesquiennial,' since 'semi-' and 'tri-' are more familiar than 'sesqui-'.