July 31 will mark the first time since 2012 that a blue moon has appeared in the sky.
It sounds like an almost magical phenomenon, but unlike when the Moon turns red during a lunar eclipse, it won't look unusual to the naked eye.
A blue moon, rather than being a visual event, is a reference to those moments when a full moon will appear in the sky twice in one month. Occurrences of blue moons are rare -- hence the idiom "once in a blue moon", which is used to describe an infrequent happening. The next one will not happen until 2018.
The first blue moon this month appeared on 2 July and another will appear in the sky tonight. They are caused by the disparity between the number of days in a calendar month -- which obviously vary -- and a lunar month, which are fixed at 29.53 days.
Unlike many space events that are visible from Earth, the blue moon will be visible from anywhere throughout the planet all through the night, meaning that as long as there isn't too much cloud cover, the phenomenon will be visible to everyone.
Visibly blue moons can also occur in the skies, but these are directly related to smoke and dust, which can be caused by events such as volcanic eruptions and forest fires.