Once in a Bluemoon 

To do something very rarely, once in a while.

For Example: “Once in a BlueMoon” DKS Getting a Win On CK

Once in a Bluemoon by CK~Hyuga July 4, 2019

once in a blue moon 

To do something very rarely, once in a while.

I don't usually hang out here, just once in a blue moon.

once in a blue moon by Roboot February 3, 2007

once in a blue moon 

To do something on a very rare occasion.

Stephan will come to the office once in a blue moon.

once in a blue moon by yurmix February 20, 2018