It doesn't happen very often, but a "blue moon" will light the night skies this month.

A blue moon is when two full moons occur in the same month, said William J. Alder, meteorologist in charge at the Salt Lake office of the National Weather Service.That will happen in this month. The moon was full May 1 and will be full again on May 31.

The overlap of calendar months and the 29 1/2-day lunar cycle only takes place about every three years, giving rise to the expression, "once in a blue moon." The last blue moon was in July 1985.

The April full moon was the "pink moon" because of early wildflowers and planting, and the May full moon is the "flower moon" because of the abundance of blossoms. "We don't have a name for the extra one so it's the blue moon," said Alder.