According to the popular definition, it is the second Full Moon to occur in a single calendar month.
On average, there will be 41 months that have two Full Moons in every century, so you could say that once in a Blue Moon actually means once every two-and-a-half years.
And 'once in a blue moon' 14 years ago - two precious babies were born!
Seems like just yesterday I was holding my first born grandchildren in my arms. They were premies at 34 weeks, and weighed 4 1/2 lbs. each. Now they are taller than I am - and starting High School next week.