|A Blue Moon isn't actually blue, but doesn't this look pretty?|
If the answer to your question is "once in a blue moon" you probably figure that it's going to be a long, long wait. But depending on the definition of "blue moon" you subscribe to, they might be more common than you imagined. One definition of "blue moon" is the second full moon in any month. We had a full moon on August 1, and now we will have a second one on August 31, thus making tonight's full moon "blue". The next blue moon of this kind will be in July of 2015.
But there is another definition of "blue moon" and that is when there are four full moons in a season. Each season normally has three full moons, but if there are four full moons in one season, the third full moon is called the "blue moon". I have no idea why the third moon is blue and not the fourth, sometimes these Old Farmers who make up Almanacs don't make a lot of sense. This type of blue moon will occur next year, in August of 2013.
So, if you are waiting for a blue moon to see your dreams come true, don't despair. They happen all the time.