"Once in a blue moon" is equal to roughly 2.7 years, in case you were wondering. And, the next one is tomorrow morning. What makes it "blue" is the fact that it will be the second full moon in July. The reason the blue moon occurs in the morning (for me) is because all full moons happen at the same time for everyone (not taking time zones into account) when the sun, Earth, and moon are aligned roughly every 29.5 days.
Tomorrow's full moon occurs at 6:43 EDT. (More here.)
I wonder what the odds are of a blue moon and a leap second occurring at the same time.