Do you know where the actual phrase "Once in a Blue Moon" came from? I do! If you take a look at your calendars, you will notice that at almost the beginning of each month, there is a full moon. This works out pretty well because the lunar cycle (the cycle of the moon) is about 29.5 days, just a few days shy of the traditional calendar. The actual term "Blue Moon" refers to the rare occurance of a full moon appearing for a second time in one month. Due to the length of the lunar cycle, a Blue Moon occurs approximately once every 2.7 years.
There ya go. You should all be suffiiently dosed with useless scientific trivia for a while. Lata!