"What is a Blue Moon?
The modern day definition of a Blue Moon is when there are 2 full moons in one Month. A full moon occurs roughly every 29.5 days and on the rare occasions when the full moon falls at the very beginning of a month there is a good chance a Blue Moon will occur at the end of the month. According to this definition the next blue moon will occur on July 31, 2015." Read more.
After tonight, Blue Moons will occur on 21 May 2016, 18 May 2019 and 22 August 2021.
Read more here.
And, when there's a Blue Moon, there's bluegrass too...
