Blue moon
There's a full moon tonight, the second this month, so this one is a blue moon. From what I've read, this occurs about 40 times in a century - less than twice a year, but it's irregular. If memory serves we didn't have a single one in 2006.

This full moon is also called a Milk Moon since it's the season in which grasses and herbs in pastures are growing rapidly, meaning that this month cows and goats will be producing lots of milk that's especially rich and high in vitamins.