We have all heard the phrase, "Once in a blue moon" to describe something that doesn't happen too often. A blue moon is commonly known as the second full moon in a month. Tonight you may not be able to see it, but the moon will be full for the second time in June meaning its a blue moon. This happens once every 2.72 years with a couple of exceptions that I will get into later. The next blue moon after tonight will be December 31st, 2009.
The only month that a blue moon can not occur us February. This is because it takes 29.5 days for the moon to go through a complete cycle and with February being only 28 or sometimes 29 days there is no way to have two full moons in that month.
With February being so short, this sets up the exception to the rule that a blue moon happens every 2.72 years. About once every 35 years there is a blue moon in January and March because February is just too short to have one.