A Blue Moon, defined as the second full moon in a single calendar month, occurs about every 2.4 to 3 years on average. Another way to say it is that there are roughly 41 Blue Moons per century. This timing happens because the lunar cycle (about 29.5 days) is slightly shorter than the length of most calendar months, making it occasionally possible to have two full moons in one month. The next "monthly" Blue Moon after now will be on May 31, 2026. There is also a "seasonal Blue Moon," meaning the third full moon in a season with four full moons, which occurs with similar frequency. Important notes:

• A Blue Moon doesn't actually look blue; the name is more about rarity than color.
• The phrase "once in a Blue Moon" expresses something rare, which is roughly once every 2.5 to 3 years.
• February cannot have a Blue Moon since the month is shorter than the lunar cycle.
• Double Blue Moons (two in one year) are rare and occur about 3-5 times per century.

This is the astronomical and calendar-based explanation for how often Blue Moons occur.