Tonight (August 31st) there is a “Blue” Moon, but our natural satellite will not actually turn blue.
This name only serves to designate the second Full Moon that occurs in the same month, something that only repeats every two or three years. This is because the synodic period of the Moon (ie, the time between two consecutive Full Moons) is 29,5 days. Therefore, a “Blue” Moon can only occur in months of 30 or 31 days (never in February), and the first one has to occur on the 1st or 2nd of that month.
This definition of a "Blue" Moon only began in 1946, as the earliest definition defined a "Blue" Moon as the third of four Full Moons in the same season of the year (usually, because of the synodic period, there are only three Full Moons in each season).
The rarity of blue moons is at the origin of the Anglophone expression “Once in a Blue Moon” (literally translated, once every blue moon, which means a rare event). The next “Blue” Moon will occur on July 31, 2015.
Author Ricardo Cardoso Reis (CAUP)
Science in the Regional Press – Ciência Viva
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