A blue moon isn't blue — the name refers to how uncommon the event is, not the moon's appearance. There are two competing definitions in use today, which causes genuine confusion about what actually counts.
The modern definition: second full moon in a month
Because the lunar cycle (about 29.53 days) is shorter than most calendar months, a full moon that falls early in a month can sometimes be followed by a second full moon before the month ends. That second full moon is popularly called a blue moon. This is the more common usage today.
The older definition: third of four full moons in a season
The original definition, used in older almanacs, is more specific: a season (the roughly three-month span between an equinox and solstice, or vice versa) usually has three full moons. When a season has four instead, the third one is called a blue moon — reserving the name for the "extra" moon so the last one could still be correctly matched to its usual seasonal name.
How rare is it?
By either definition, blue moons happen roughly every two to three years — hence "once in a blue moon" as a phrase for something infrequent, though not vanishingly rare.
Does the moon ever actually look blue?
Very rarely, yes — but not from this astronomical phenomenon. Genuinely blue-tinted moons have been observed after major volcanic eruptions or large wildfires, when particles of a very specific size in the atmosphere scatter red light more than blue, altering the moon's apparent color. That's an atmospheric event, unrelated to the calendar-based "blue moon" described above.
Check the 2026 full moon calendar to see this year's blue moon date.