Once in a Blue Moon

What does this idiom mean? "Not very often". "De higos a brevas", "de uvas a peras", "muy de vez en cuando", "una vez en un millón" ... Why?

The modern day definition of a Blue Moon is when there are 2 full moons in one month. A full moon occurs roughly every 29.5 days and on the rare occasions when the full moon falls at the very beginning of a month there is a good chance a Blue Moon will occur at the end of it. According to this definition we have a blue moon today.

For exact times of the Full and Blue Moons see the Full Moon Calendar.