Blue Moon -- changing definitions / one to see and notice!

from EARTH SKY dot-com : There are different definitions for Blue Moon. By popular acclaim, Blue Moon refers to the second of two full moons in a single calendar month. A Blue Moon might also be the third of four full moons in a single season – a season being defined as the time period between a solstice and an equinox, or vice versa. Or, someday, you might see an actual blue-colored moon. A Blue Moon falls on the night of August 20-21, 2013. It’s a Blue Moon by the seasonal definition, that is, the third of four full moons to take place in a season, in this case between the June 2013 solstice and September equinox. The last Blue Moon by this definition happened on November 21, 2010. Every month typically has a full moon (although sometimes February doesn’t have a full moon at all). In fact, our word for “month” comes from the word “moon.” Most of the time, the names for full moons coincide with particular months or seasons of the year. So whether you define a Blue Moon as the second full moon in a month – or the third full moon of four in a season – the name Blue Moon accounts for times when there are more full moons than is ordinary. The term once in a blue moon used to mean something rare. Now that the rules for naming Blue Moons include so many different possibilities, Blue Moons are pretty common.