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.

[El_Phil]

It appears the number I pulled was counting the current definition of blue moon and the old definition of the third full moon in a season of four full moons. Which apparently was very important for the old lunar Christian ecclesiastical calendar. Thirteen moons a year would throw it all out so ignoring one of the moons as a 'Blue Moon' kept it all together.

You learn something every day, even if it is of no real use, then again the same could be said of my uni lecture today. 'Dredging hydraulic transport technology.' Less exciting than it sounds trust me