July ends with a blue moon
The first full moon of July occurred on the 1st, and now a second is coming on July 31.
A month with two full moons happens every few years, but it’s relative infrequency gave birth to the expression, “Once in a blue moon”, which means something that happens rarely.
Typically, there is only one full moon each month. Because it takes the moon about 29½ days to circle the Earth and most months are 30 or 31 days long. Its is possible to fit two full moons in a single month, although it only happens about every 2½ years. The last blue moon was August 31, 2012, almost 3 full years ago.
The literal meaning of a blue moon is when the moon appears to be unusually bluish, which is a rare event. The effect can be caused by smoke or dust particles in the atmosphere.