In recent times, people have taken to calling a full moon a blue moon based on the Gregorian calendar. By this use of the term, a blue moon is the second of two full moons to occur in the same calendar month. This definition of blue moon originated from a mistake in an article in the 1946 Sky & Telescope magazine, which failed in an attempt to infer the earlier definition used in the original Farmer's Almanac (see above). It was helped to popularity when a Trivial Pursuit question used this as a source for one of its questions. Sky & Telescope discovered the error nearly 60 years later and the magazine printed a retraction and correction. But by the time the correction came, the calendar definition had already come into common use. Because it is so much easier to understand, the "mistaken" calendar-based meaning has stuck.
Calendar blue moons occur infrequently, thus using the saying once in a blue moon to describe a rare event is still meaningful. The infrequency of extra moons in a month is because the length of the Gregorian calendar months are all very close to the length of the 29.5306 day period (on average) of the moon's phases: the synodic month, or lunation. Second full moons are possible because every month except February is longer than this period by 1 or 2 days, but the odds are low. One calendar blue moon occurs � on average � every 2.72 years. The next two calendar blue moons (based on UTC) will be on June 30, 2007 (but May 31, 2007 in the Western Hemisphere; see below); and December 31, 2009. Because February (according to UTC) will have no full moon in 2018, January and March will each have a calendar blue moon that year.