A phenomenon will occur tonight that only happens once in a blue moon - literally.
If you ever wondered where that expression came from, step outside tonight. For the second time in a single month, there will be a full moon.
This is a slightly rare event that only happens around once every three years. The next time two full moons will occur in the same month will be in July of 2015.
Don't be prepared to see an actual blue-tinted moon, though. It's unclear where the expression "blue moon" comes from, but it was coined to refer to the third full moon during a season in which there are four full moons.
Because this event only happens once every three years or so (2.7 to be exact), farmers coined the phrase "blue moon" at least seven decades ago, likely just to denote the fact that it is something you don't see that often.
Somehow, that led to the common phrase of something rare happening "once in a blue moon." It has the perfect meaning, really, since even though the celestial event doesn't happen that often, it is reliable.
The expression isn't the same as saying that something will happen when pigs fly, for example. To say something happens "once in a blue moon" means that it does happen, but you shouldn't hold your breath waiting for it.
So, again, even though the sky won't look any different to the average viewer, those who are in the know will realize that it is still a special event.
* Note: The term "blue moon" can refer to either the third full moon in a season with four full moons, or the second full moon in a month.