A blue moon, defined as the second full moon occurring in a single calendar month, typically happens about every 2.4 to 2.5 years on average. This rare event roughly occurs 41 times per century, meaning it happens about seven times every 19 years. This is because the lunar cycle is about 29.5 days, and occasionally two full moons can fit within one month since most months have 30 or 31 days (February never has a blue moon).