When we have two full moons in a single month, the second one is called a blue moon. It is a fairly infrequent phenomenon. This Saturday, March 31, is not only a blue moon … it’s the second blue moon we will experience this year (the first happened on January 31st). The last time we had two blue moons in one year was 1999, and it won’t happen again until 2037 — so take a moment to glance up at this unusual occurrence.
A blue moon is not colored blue. Folklore experts think the term has been around for about 400 years, and came from the fact that moon does not look blue. People would use it as a way of saying never: “I’ll marry you when the moon turns blue!” But in 1883, the volcano Krakatoa exploded and filled Earth’s atmosphere with dust that acted as a filter, causing the moon to look blue or green. After that the meaning shifted slightly, from never to rarely … so we say something unusual happens “once in a blue moon.”
Another random factoid about blue moons is that one can never occur in February. The lunar cycle gives us a full moon once every 29.5 days — and since February usually has 28 days and at most 29, it is out of luck.