About five years per century end with a December blue moon, if I have my numbers right -- 31 days in December, ~29.5 day moon cycle, so you need a full moon in the first 36 hours or so of December to have another full moon in the same month. Over the long term full moons scatter randomly, so the odds of December having a blue moon is about 3/62.

About every two hundred years or so, a decade ends in a blue moon.