Once in a blue moon.

[Alneyan]

I would say a blue moon is a bit more unlikely, more like every two or three years: there is roughly 354 days between 12 full moons (29.5 days per full moon, again on average), leaving 11 days on the average year for a 13th full moon, so the odds of it would fall somewhere between 1/2 and 1/3.

Though I haven't checked their calculations (I gather they are much better at maths than myself), this calculator gives 9 blue moons between 2004 and 2024.