Blue Moon
The term Blue Moon means something that happens rarely, as in "once in a blue moon". It is rare that in one calendar month you would see a second full moon or "blue moon". This is an event that is seen as absurd and can never occur. Although the moon is never actually "blue", unless alot of dust is kicked up into the atmosphere, then the moon can almost appear blue. Some events in history that have caused this effect and made the moon appear as blue include: The eruption of Krakatoa in 1883, late Indian monsoons in 1927, and forest fires in Canada in 1951.

*This from another website visitor:
Blue moon is actually an astronomical term when there are two full moons in a month. Also, when there are two new moons in a month, that's called a Black Moon. (来源：英语麦当劳－英语快餐EnglishCN.com)