What is a “Blue Moon?” According to the Farmer's Almanac, you can look up in the sky on August 31, and you will see one for yourself.
Blue Moons occur, on average, once every 2 1/2 to 3 years. Perhaps this is why we traditionally describe an unusual event as happening “Once in a Blue Moon,” an expression that was first noted in 1821.
For the longest time nobody knew exactly why the second full Moon of a calendar month was designated as a Blue Moon. One explanation connects it with the word “belewe” from the Old English, meaning, “to betray.” Perhaps, then, the Moon was “belewe” because it betrayed the usual perception of one full Moon per month.
Here's some mood music to celebrate the evening: