People familiar with the term Blue Moon usually think of it as the second of two full Moons in the same calendar month. But the Blue Moon can also refer to the third of four full Moons to occur in one season.
Photo on right: creepyhalloweenimages
Seasonal Blue Moon on August 21, 2013
Did you know, though, that it's possible for ONLY two full Moons to fall in a single season? Yes, much to my surprise, it can happen during a Northern Hemisphere winter (or a Southern Hemisphere summer), though it is extremely rare. I learned from obliquity.com that ONLY two full Moons took place in between the December 1961 solstice and the March 1962 equinox:
Full Moon: December 22, 1961 (00:42 Universal Time)
Solstice: December 22, 1961 (02:19 UT)
Full Moon: January 20, 1962
Full Moon: February 19, 1962
Equinox: March 21, 1962 (2:30 UT)
Full Moon: March 21, 1962 (7:55 UT)
In this instance, four full Moons cram into the previous season (between the September 1961 equinox and December 1961 solstice), and into the following season (between the March 1962 equinox and the June 1962 solstice). Seasonal Blue Moons fell on November 22, 1961, and May 19, 1962. (Note that a monthly Blue Moon also happened in 1961, on January 31.)
Because the full Moons from December 1961 to March 1962 closely aligned with apogee - the Moon's most distant point in its orbit - the time period between successive full Moons was longer than average. And with the year's shortest season being from the December solstice to the March equinox, the stage was set for a two-full-Moon winter season.
Lunar Perigee and Apogee Calculator
Asking obliquity.com when only two full moons will fall in one season again, Dr David Harper answered, "Between 1962 and 3000, it happens only four more times, in the winters of 2314/5, 2333/4, 2686/7 and 2705/6. In each case, as in 1961/2, there is a Full Moon less than five hours before the December solstice, and there are four Full Moons in both the preceding autumn and the following spring."
You may discern two lunar cycles at work when it comes to realigning Moon phases with the seasons: the 19-year Metonic cycle and the long period cycle of 372 years.
Seasonal Blue Moons - 20th and 21st Centuries
Solstices & Equinoxes from 1900 to 2099
Equinox & Solstice Calculator
Catalog of Moon Phases