Here's an intriguing article with lots of juicy linguistic and etymological tidbits pertaining to this phenomenon:

http://www.cnn.com/2004/TECH/space/07/30/blue.moon/index.html


>The phrase "Once in a blue moon" was first noted in 1824 and refers to occurrences that are uncommon, perhaps even rare. Yet, to have two full Moons in the same month is not as uncommon as one might think. In fact, it occurs, on average, about every 32 months. And in the year 1999 it actually occurred twice in three months.

For the longest time no one seemed to have a clue as to where the "Blue Moon Rule" originated. I myself once suggested that the rule might have evolved out of the fact that the word "belewe" came from the Old English, meaning, "to betray." Perhaps, I suggested, the second full Moon is 'belewe' because it betrays the usual perception of one full moon per month.

It was not until the year 1999 that the origin of the calendrical term "Blue Moon" was at long last discovered. It was during the time frame from 1932 through 1957 that the Maine Farmers' Almanac suggested that if one of the four seasons (winter, spring, summer or fall) contained four full moons instead of the usual three, the third should be called a "Blue Moon."<