The phrase "rare occasions" refers to events or situations that occur infrequently, not often, or only occasionally. It describes a situation where something happens or is experienced rarely, making it special and unique when it does happen.
I found myself only remembering to take photos outside investigations on rare occasions when absolutely no zombies were around. (cheatcc.com)
Genetic testing is not routinely done or recommended, and genetic causes are searched for only in rare occasions when other causes can not be found. (sciencedaily.com)
These are very rare occasions where we spend many hours searching and contacting potential sites and can not find anyone is appropriate and who is willing or qualified to participate. (josephinayatarpr.com)