What is the meaning of "Once in a Blue Moon"? 🤔

A Blue Moon 🌑 refers to the second of two full moons 🌕 to fall within a single calendar month. According to Google the frequency of a blue moon is f=1.167×10-8 hertz thus converting to years:

Once in Blue Moon 🌑 ~= 2.7 years pic.twitter.com/YdLoBPvQsN

— Fermat's Library (@fermatslibrary) March 8, 2019