Next Blue Moon Countdown

Where the phrase actually comes from

"Once in a blue moon" predates any astronomical definition — the "two full moons in a month" meaning is a 20th-century reinterpretation, originally from a 1946 magazine article that misread an older Farmers' Almanac seasonal naming convention. It stuck anyway, and is now the commonly understood definition.

Why it happens roughly every 2-3 years

A full lunar cycle (29.53 days) is shorter than every calendar month except February, so most months only fit one full moon. Occasionally the timing lines up so a month starts with a full moon early enough that a second one squeezes in before the month ends — this can never happen in February, since it's too short to ever contain two.