Once in a Blue Moon ... is a common way of saying not very often, but what exactly is a Blue Moon?
According to the popular definition, it is the second Full Moon to occur in a single calendar month.
The average interval between Full Moons is about 29.5 days, whilst the length of an average month is roughly 30.5 days. This makes it very unlikely that any given month will contain two Full Moons, though it does sometimes happen.
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.
