Blue Moon

Folklore doesn’t have to be old to be popular. Consider, for example, the modern definition of Blue Moon: the second full Moon in a calendar month. Although that definition began as a mistake, it became popular more than two decades ago and has ingrained itself in skywatching lexicon.
And by that definition, there’s a Blue Moon the next couple of nights. There was a full Moon on July first, and there’s another early tomorrow morning — the second full Moon of the month.
Blue Moon already had several definitions, including the thirteenth full Moon in a calendar year, and the fourth full Moon in a calendar quarter. The phrase could also be taken literally. Under some rare atmospheric conditions — when there’s a layer of ash high in the air, for example — the Moon can actually look blue. And the phrase “once in a blue moon” is unrelated to the Moon itself, and simply means that an event is rare.
The “second-full-Moon-in-a-month” definition entered the culture in the 1980s, thanks to a mist