Once in a Blue Moon

Did anyone else catch it last night? "What?" you may ask. The Once-in-a-Blue-Moon phenomenon.

Apparently, the moon is not blue. It rather refers to the fact that two full moons rarely occur within the same calendar month, since a lunar cycle is ~29.5 days, and months average 30 days. With a full moon already having occurred 2 December, and one occurring last night (31 December), we will not see another Once-in-a-Blue-Moon for another 2 1/2 years (August 2012), and not another Once-in-a-Blue-Moon on New Year's Eve until 2028. Set your alarms now!

Apparently, according to my source, the expression comes from a 1528 book published in England, that had the following reference:

"If they say the moon is blue, We must believe that it is true."

Well, as you can see, the moon -shot out my back door- was blue for me.