re: #497 SixDegrees

And as others have already said, it’s commonly the second full moon occurring within a single month. Although the history of the phrase is more muddled; see the link on my previous post.

Yeah, I have heard a blue moon as a 13th full moon of the year.
So that would be true as well, but 2 full moons in one month seems to be the most common among astronomers, and to take that pattern, its 3.4 blue moons per year on average.
Yes I am bored. ;)