A blue moon is the second full moon in a calendar month, or the third of four in a season; it is not blue, and comes every two or three years.

Because lunations run 29.5 days, a month of thirty or thirty-one can hold two full moons, which happens every two to three years. The colour has nothing to do with it; the phrase is old and meant 'rarely'. A moon can look blue after a great fire or eruption, when smoke of one particular size scatters the red — that is rarer still.

The month kind is the popular one; Sol lists both, with the year's dates.