Blue Moon Astronomy

NASA.Gov

An old-school definition of a Blue Moon is when there are four full moons in a season instead of three - with a season being between a solstice and equinox.  The third of the full moons is traditionally called a Blue Moon.  On average, this occurs once every 2.7 years.  The next seasonal Blue Moon will be August 22, 2021.

The current (modern) definition of a Blue Moon is when two full moons occur within the same calendar month.   

Tomorrow's Halloween full moon - a Hunter's Moon - is a repeat of the full moon that appeared on October 1.  

On rare occasions you might be lucky to be alive to witness both of these types of Blue Moon appearing in the same calendar year.  The conditions for this require thirteen full moons between successive December solstices - for a seasonal Blue Moon.  And thirteen full moons in one calendar year - for a monthly Blue Moon.

The next time these conditions align will be 2048.  In that year there will be a monthly Blue Moon on January 31 and a seasonal Blue Moon on August 23.  Mark your calendar.

Lastly, Blue Moons defined as above are not blue in color.  If you were to see a real blue-colored moon it would be because of very poor air quality conditions that have a great deal of particulate matter in the atmosphere.  If you see a photo of a blue-colored moon it likely a result of Photoshop.

You can learn more here.