According to the Farmers’ Almanac, a blue moon is either an additional full moon in a given season (there are normally three, or 12 per year), or a second full moon in a single month (since the moon cycle is typically 29 days). The phenomenon only happens about once every two and a half to three years, so it’s a pretty uncommon occurrence. This year, the blue moon happens to fall on October 31, Halloween, which only happens once every 19 years.

Even though the Halloween blue moon is rare and exciting, don’t miss out on this year’s harvest moon, which falls on October 1.