When ever there are four Full Moons in a quarter, as there will be this Autumn - the third Full Moon ( the one at the moment ) in that period is known as a Blue Moon.
The phrase: Once in a Blue Moon, relates to the rarity of an event or of something happening. When in actual fact, a blue moon occurs once annually, in about two out of every three years. There will be no Blue Moon in 2011, so this will be the last chance to actually do something 'Once in a Blue Moon' until 2012.
The Algonquian Indian name for the November Moon, is the Beaver Moon :