There has been a bit of hype recently about the supposed blue moon today. This is actually not a blue moon however since the actual blue moon can only occur around four specific days of the year give or take a day or so.
The possible dates are February 19, May 20, August 22 and November 21. These are approximate give or take a day but no other months can have a blue moon since they signify the third of four full moons in a season and the seasons start at the times of the winter and summer solstice and the spring and fall equinox.
The purpose of the blue moon is to have the names of the seasonal moons consistant, having three named moons per season. I would consider the full moon today as the Corn Moon. It's a little early for the harvest moon here since it's so warm in August. The next moon would be the Harvest Moon, the time when the harvest should probably come in. The names of the moons differ depending on who is naming them, where they live and what is important to their cluture.
The Jewish calender kept this by having an extra month every few years at the end of the year. Thus on years with a blue moon during the sacred Jewish calendar, which starts with the new moon before the full moon after the spring equinox, the month of Veadar is added to keep the months straight. Each month being from new moon to new moon.
As mentioned in a previous post the idea of a blue moon being the second moon in a month doesn't work because of the famed double blue moon, a second moon in January followed by a second full moon in March. This throws the naming all out the window since there would be no full moon in February and two blue moons would mean a named moon would be missing.
The next real blue moon is on August 21, 2013. The other three full moons that season are on June 23, July 22 and September 19. This means that next year's harvest moon will be October 19 which is almost a month later than this year.
(The above picture is not a full moon but was taken a couple days early on a clear night.)