They happen about…once in a blue moon
Blue moons aren’t actually blue, but they are as rare as the saying goes.
The second full moon in a month is commonly referred to as a blue moon—we had a full moon on July 2, and we’ll have another this Friday on July 31—but there’s a more complicated definition according to the U.S. Naval Observatory:
“The traditional definition is that of the third Full Moon to occur in an astronomical season in which four Full Moons fall. While somewhat more cryptic than the ‘second-of-the-month’ variety, this scheme helped farmers and other people who used traditional Full Moon names to keep the proper names in synch with the occasional “extra” Full Moon that occurs about every 2.6 years. The “controversy” over the proper definition of the “Blue Moon” dates back to an erroneous discussion of the phenomenon that was originally published in 1946 and subsequently re-discovered in 1999.”