We often use the idiom 'once in a blue moon', meaning 'very rarely', and tonight we have that rare phenomenon, a blue moon. A blue moon in modern parlance is the second full moon in a single calendar month. That is a rare event because a full moon cycle is 29.5 days. Tonight's moon is the second full moon of July; the first was on 1st July. The next blue moon won't be until January 2018.
I'm looking at the moon now, and it certainly isn't blue. However, according to the OED, the original meaning of blue moon was a moon that appears to be blue, owing to the presence of dust particles or smoke in the atmosphere. This meaning gave rise to the idiom 'once in a blue moon'. Then, in the US in 1937, the Maine Farmer's Almanac mentioned another meaning of blue moon - the third full moon of a season that, exceptionally, contains four full moons. Farmers used to call each of the moons of the year by a special name (indeed, such poetic names for the moons have been used for centuries): Moon after Yule, Wolf Moon, Lenten Moon, Egg Moon, Milk Moon, Flower Moon, Hay Moon, Grain Moon, Fruit Moon, Harvest Moon, Hunter's Moon and Moon before Yule. When there was an extra full moon in the season, they needed a thirteenth name. See this page for more information on moon names. The current meaning of blue moon, which relates to the number of full moons in a month, rather than a season, seems to have come about in the 1940s because of someone misquoting the Maine Farmer's Almanac explanation.
For more on tonight's blue moon, see this BBC article.