If you promise people that you only do things "once in a blue moon" then today is the day to make good on that promise. Tonight, as the moon rises in the sky, we will be privy to what is called a blue moon -- we won't get the next one until July 2015.
Sadly a blue moon is not actually blue, and instead looks very similar to a normal full moon; that said, on rare occasions (often in conjunction with volcanic eruptions and forest fires) the moon can appear blue. By definition a blue moon is the second full moon to rise in one calendar month. We had a full moon on Aug. 1 and we now have one again on Aug. 31.
Today's full moon officially occurred at 9:58 am, but you won't be able to really check it out until this evening. Tonight the moon will rise in the eastern sky at 6:30pm, so start your moon viewing then. In addition to referring to the second full moon in a month, a blue moon can also refer to the third full moon in a season with four full moons. A typical season only has three moons.
In pop culture, blue moons have long been used as a way to mourn lost love. Crooner Elvis Presley made the song "Blue Moon" a global hit back in 1956.
If you have caught the blue moon bug, Slooh.com will show live video from telescopes around the world starting at 6 pm EST on Friday.
A very user friendly (and incredibly interesting!) blue moon explanation can be found here: