The definition of what a blue moon is has changed in recent history. Full moon names were given to each full moon in a given season, so blue moons were a 4th full moon in the given season, for which there would be no name.
But in a 1946 Sky and Telescope article, James Hugh Pruett misinterpreted this to mean twice in a month, ignoring the season component. This meant that the definition of a blue moon was changed to the second blue moon in any given month. This has since become the popular definition.
Full Moon 2018
Full moon on January
Tuesday, January 2, 2018
02:25 AM
Full Wolf Moon
Full moon on January
Wednesday, January 31, 2018
01:27 PM
Blue Moon
Full moon on March
Friday, March 2, 2018
12:52 AM
Full Worm Moon
Full moon on March
Saturday, March 31, 2018
12:37 PM
Blue Moon
Full moon on April
Monday, April 30, 2018
12:59 AM
Full Pink Moon
Full moon on May
Tuesday, May 29, 2018
02:20 PM
Full Flower Moon
Full moon on June
Thursday, June 28, 2018
04:54 AM
Full Strawberry Moon
Full moon on July
Friday, July 27, 2018
08:22 PM
Full Buck Moon
Full moon on August
Sunday, August 26, 2018
11:58 AM
Full Sturgeon Moon
Full moon on September
Tuesday, September 25, 2018
02:54 AM
Full Harvest Moon
Full moon on October
Wednesday, October 24, 2018
04:47 PM
Full Hunter's Moon
Full moon on November
Friday, November 23, 2018
05:41 AM
Full Beaver Moon
Full moon on December
Saturday, December 22, 2018
05:50 PM
Full Cold Moon