My astronomer friend tells me that tonight we will see a "blue moon". However, he doesn't mean that the moon will literally appear blue.
Blue moon is the name given to an irregularly timed full moon. Most years have twelve full moons which occur approximately monthly, but each calendar year contains those twelve full lunar cycles plus about eleven days to spare. The extra days accumulate, so that every two or three years there is an extra full moon (making 13). The extra moon for that year is called a blue moon.
It is where the saying "once in a blue moon" which describes a rare event comes from.
Farmers define a blue moon as one that occurs in a season where there are more than three. Others define the second full moon that occurs in a given month as a blue moon.
Whether tonight's moon is a blue moon or not depends on whether you use the Farmer's Almanac definition or the "Calendar Blue Moon" definition.