
Full moons happen about every 29.53 days (the precise average period is 29.53059 days,but it's not always that exact interval because there are variations through the year' as the Earth-moon distance, Earth-sun distance, and sun-moon distance change)So you'll get a blue moon in a 31-day month if and only if the first full moon occurs less than 1.47 days into the month, which'll happen, on average, 1.47 / 31 of the time... or about 5% of 31-day months.
Similarly, in a 30-day month you get a blue moon every .47/30, or 1.5%.
These chances work out to a blue moon every 19 months, on average. It varies widely, however, and you can have several years without a blue moon at times.
You can't ever have a blue moon in February, since at most it has 29 days.

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