Strangely entering this into google got me the exact information I wanted. A blue moon is the second full moon in one calendar month. They happen on average once every 41 months, or roughly every two and a half years.
By a crazy coincidence we’ve just missed a blue moon here in England by two days (was on 31st of July). I say this is a crazy coincidence as the subject came up in the car on the way home yesterday, whilst noticing the moon being unusually red.
The next blue moon will be in 2007 on the 30th of June, if you are curious to know more about blue moons and when they will next occur, all the information I have here was from Once in a blue moon.