Blue moon is a term that has two different meanings. In the astronomical sense, a blue moon is the second full moon that appears in a single calendar month. Since most months have only one full moon, a blue moon occurs once every two and half years on average. Blue moons are not actually blue in color, they appear the same as any other full moon. The term "blue moon" likely derives from the Old English phrase "belewe mone," which means "betrayer moon" or "deceiving moon" This is because the appearance of a second full moon in a month was considered a rare and unusual event, and was thought to bring misfortune.

The other definition of a blue moon is a full moon that occurs in November. This definition is based on the fact that the full moon in November is traditionally associated with the beaver moon. The beaver moon gets its name from the fact that beavers are said to be particularly active during this time of year. For some people, the term blue moon is simply another name for the beaver moon.

So, while a blue moon is not actually blue in color, it is still a unique and interesting phenomenon.