Blue moons typically occur about once every two to three years. This frequency applies both to the common definition of a blue moon as the second full moon in a calendar month and to the older definition as the third full moon in a season with four full moons

. More specifically:

• A blue moon happens roughly seven times every 19 years, which averages to about once every 2.7 years

• On average, a blue moon occurs once every 33 months or about 41 times per century

• Occasionally, two blue moons can happen in the same calendar year, which is rarer and occurs about four times per century

• The next double blue moon year will be 2037

The phrase "once in a blue moon" reflects the relative rarity of this event, although blue moons are more common than the phrase suggests. Actual blue- colored moons are extremely rare and caused by atmospheric conditions, not calendar phenomena

. In summary, blue moons appear approximately every two to three years, with some variation depending on the exact definition used and the lunar cycle