The next blue moon is August 31, 2012, but what does the term "blue moon" actually mean?
It doesn't actually mean a moon that is the colour blue (though that would be stunning). The term "blue moon" is actually very inconsistent: If you're reading a traditional farmer's almanac, it will tell you that there's a blue moon only once every two or three years, when a "season" has four moons instead of the normal three (farmers traditionally measured seasons as three months, with one moon for each month). Every moon was given a folk name, and this extra, otherwise nameless moon was called a blue moon.
Starting in the early 1980s, we also began to define blue moons as the second full moon to occur in one month, which happens more frequently than in the previous definition (every year or two, usually). This definition is perhaps more common now because it's more definable and syncs with our Gregorian calendar system.
Essentially, a blue moon is an "extra" full moon. We think the term comes from the Middle English word belewe, which meant both blue and betrayer. Lent is calculated based on lunar cycles, and if a moon came too early, Medieval Clergy called it the "betrayer" (belewe) moon and scheduled Lent for the moon after.
Blue moons are a special time; take advantage of this special energy by focusing on ritual or magic that needs extra intention, charge magical tools, or create a ritual to reach out to your goddess, recognizing that this is also a time of divinity and magic. The rarity of this moon means it's worth taking extra care in our celebration and acts of magic on this night.