Phrase of the week - Once in a blue moon!

http://www.sciencealert.com/a-blue-moon-and-a-red-mars-will-pair-up-this-week-here-s-how-to-watch

Phrase of the week - Once in a blue moon!

This weekend there will be an actual blue moon in our skies. In English we use this expression to explain that an event is not usual, hardly ever happens. 

For example: 
A: How often do you go to the gym? 
B: Since I've had children I go once in a blue moon these days!

‪#‎onceinabluemoon‬ ‪#‎astronomy‬‪#‎englishidioms‬