Blue moon

Blue moon, the second full moon in a calendar month. The period from one full moon to another is about 29 1/2 days, so when two occur in the same month, the first of these full moons is always on the first or second day of the month. February, which has only 28 days (29 days in leap years), can never have a blue moon. Months with 31 days—January, March, May, July, August, October, and...