How many times have you heard the phrase "once in a blue moon"? Well, it's more than just an expression - a blue moon is the name given to an actual phenomenon. Here's how it works.
A full lunar cycle is a little over 28 days long. However, a calendar year is more than that, which means that during some years, you may end up with thirteen full moons instead of twelve, depending on where in the month the lunar cycle falls. This is because during each calendar year, you end up with twelve full 28-day cycles, and a leftover accumulation of eleven or twelve days. Those days accumulate, and so about once every 28 calendar months, you end up with an extra full moon during the month.
In folklore, the monthly moon phases were each given names that helped people prepare for various types of weather and crop rotations. Although these names varied depending on culture and location, they generally identified the sort of weather or other natural phenomenon that might take place in a given month. The term "blue moon" was given to the second full moon to appear in a calendar month.
Although there is no formal significance attached to the blue moon in modern Wiccan and Pagan religions, you can certainly treat it as an especially magical time. Think of it as a lunar bonus round. In some traditions, special ceremonies may be held -- some covens only perform initiations at the time of a blue moon.
Examples: On the night of the blue moon, Willow's coven initiated two new Seekers.