A blue moon, defined as the second full moon in a single calendar month, occurs roughly every 2 to 3 years on average. This frequency comes from the lunar cycle of about 29.5 days being shorter than most calendar months, allowing two full moons to occasionally fit into one month. Statistically, blue moons happen about 41 times per century, or about once every 33 months on average. The phrase "once in a blue moon" reflects this relatively uncommon but not extremely rare occurrence.

There is also a seasonal definition of a blue moon, which is the third full moon in a season that has four full moons instead of the usual three. This type of blue moon occurs with a similar frequency, roughly every 2 to 3 years.

Notably, February can never have a blue moon because its short length (28 or 29 days) cannot fit two full moons. Some years may experience two blue moons (a "double blue moon") which is rarer and happens only a few times per century.

In summary, blue moons usually appear about every two to three years either by the monthly or seasonal definition.