The phrase "rare occurrence" means that something doesn't happen very often or it is not common.
Sentences with «rare occurance»
Which is the planets having one of its lovely warm spells which is an all too rare occurance in earths geological records. (desmogblog.com)
We had a fantastic January with Mkhitaryan, Aubameyang signings and Ozil signing new contract, but this was a very rare occurance where Wenger felt pressure due to our circumstances. (justarsenal.com)
The chief whip gave a live interview with the BBC this afternoon, an extraordinarily rare occurance which goes some way to dramatising the strange and nervous mood permeating Westminster. (politics.co.uk)