You've all heard the term "once in a blue moon" before, usually in reference to something that almost never happens. In reality, a blue moon is defined as a second full moon in one calendar month. Because there are roughly 33 lunar months for every 32 calendar months, the blue moon actually occurs slightly more often than once every three years. In 2007, there will be 13 full moons, so we will all get to see a blue moon this year. What's strange is that the full moon you consider to be a blue moon will be different depending on how you measure time. If you live in the United States, you will see a full moon at 9:04 ET on May 31st. Because there is also a full moon earlier that month -- on May 2nd at 6:09 ET -- the full moon on May 31st will be your blue moon. If you live in Europe, that 2nd full moon will actually occur in the early hours of June 1st, so the full moon on June 30th is your blue moon.