The Full Moon of May is called the Milk or Planting Moon, and this month you’ll have two of them, an event that occurs only “once in a blue moon!”
After May 18 2019, the next double-mooned month will be October 2020. The time between 1 full Moon to the next, as seen from Earth, is 29 1/2 days. This is called the moon’s synodic period, which is different from the true period (called sidereal) by 2 1/6th days.
Although the actual Full Moon occurs 2 1/6 days earlier, we see it as such later because in the meantime the Earth has moved along in its own orbit. There’s a 21/6 day drag. The name Milk Moon is from the Anglo-Saxon word for the month: Thrimilce.
Cows can be milked 3 times a day in May. Since the Roman festival of the unhappy dead and that of the goddess of chastity was celebrated in May, this month has long been considered an unlucky time for marriage.
This probably explains the popularity of June weddings.
Keep in mind that a true Blue Moon occurs only when small particles in the upper atmosphere (from volcanic eruptions, forest fires, etc) cause the Moon to appear blue. This, however, is about as rare as two Full Moons in the same month, hence the saying.