Blue Moon July 31st 2015

So . A blue moon . What is it ? As I understand what I've read, there are four seasons (spring, summer, autumn, winter) each with three months and each of those months has a full moon . A fourth moon shows up from time to time .
It is the second of two moons in one of those months . Clear as mud ? To me too . And that 'blue' business ? It's some rare atmospheric dust that affects its colour from time to time . So I guess that makes it happen ... once in a blue moon .

The moon. (cropped)

Reflection on the Fraser River. (cropped)

Skytrain cables. (cropped)

Bonus: little sparrow at the Starbucks down the hill. (cropped)

The next blue moons:

May 21, 2016
May 18, 2019
August 22, 2021
August 12, 2024

I guess they are pretty rare .