You know the phrase once in every blue moon. It means a rare event that doesn’t happen very often. A blue moon is the second full moon in the same calendar month. Today’s full moon (July 31) is a blue moon. We had a full moon back in July 2 and the second one today. The next blue moon will not happen again until January 2018. Why do we have blue moons? A lunar month is 29 days long, while our monthly calendar is 30 to 31 days, with February being the sole exception. So, it’s not unusual at all for two full moons to fall within the same month. It just doesn’t happen so often. More like every 2.5 years.