Once in a Blue Moon

It takes about 27 days for the Moon to circle around the Earth, but due to Earth's own circular movement around the Sun it takes few more days for the Moon to complete the full circle - so the Moon will be in the exact same location (with respect to Earth) in about 29 days.

Thus, the Moon cycle (or month) is little smaller than our normal calendar month. So normally we would see full Moon once in a month - totalling 12 full moons in a year, but due to this mis-match between Lunar and Solar cycles, after about every two/three years there would be 13 full Moons in a year. So one calendar month will end up having two full moons. The second of the two full Moons that occurs in the same calendar month is called Blue Moon.

Due to the rarity of such events - the common phrase came into existence: once in a blue moon.
(The word Moon comes from a Latin word mensis which means something like "measure".)

By the way, it always fascinates me when I think about the relative sizes of Sun and Moon and their distances from Earth - and still they look exactly of same size from Earth (which also makes full solar eclipeses possible). How big of a coincident is that, huh?