Now of course this has nothing at all to do with the moon turning blue, lol. At least not unless a volcano is erupting in your neighbourhood or perhaps a massive forest fire nearby. A blue moon occurs once every 2 or 3 years, and is simply when you have 2 full moons in one calendar month. The lunar cycle is of course 12 cycles plus 11 days in one of our calendar years, meaning that every 2 and a bit years we have 13 full moons in a calendar year.
This years blue moon occurs tonight, Friday August 31st, 2012.
And now you know where expression "once in a blue moon" comes from, meaning something that doesn't happen very often.