If something happens once in a blue moon, it is extremely rare.

For example:

My sister lives in Alaska, so I only see her once in a blue moon.

Once in a blue moon, he’d call, but for months at a time, we heard nothing.

That company puts on a good performance only once in a blue moon.

Peter only comes out for a drink once in a blue moon now that he has kids.

He tries to cook, but it’s once in a blue moon, so we usually eat out

Tradução: uma vez na vida- muito raramente- quase nunca- uma vez na vida, outra na morte.