mid-autumn festival
A blue moon is where a calendar month has two full moonsReuters

July 2015 is set to experience a rare phenomenon – two full moons in one month.

It will be the first time since August 2012 that we experience two full moons in the space of a month. The first has already passed on 1 July, but the second will come on the last day of July.

This only usually comes around every three years or so, hence the term "once in a blue moon".

However, if you want to see a slightly discoloured moon, you are out of luck. The name does not apply to the colour in these circumstances, but rather a corruption of the term "belewe moon", or "traitor's moon". A slightly blueish moon can occur if the atmospheric conditions are right, but it is unlikely to be on 31 July.

Full moons are usually separated by 29 days, while most months are 30 or 31 days long; so it is possible to fit two full moons in a single month.

The next blue moon will come about in 2018, so moon lovers and werewolves need to take advantage of the upcoming one.