If there is something that you do “Once in a Blue Moon” tonight is the night to do so. The phrase traditionally means that there has been a full moon twice in a month, which is what is occurring tonight.
So let your hair down and enjoy the “Blue Moon” tonight.
The phrase “Once in a blue Moon” was first noted in 1824 and refers to occurrences that are uncommon, though not truly 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 occurred twice in a span of just three months!
For the longest time no one seemed to have a clue as to where the “Blue Moon Rule” originated. Many years ago in the pages of Natural History magazine, I speculated 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.” via Yahoo! News.