They weren't joking when they came up with the saying about things only happening once in a blue moon.
Tonight will be the first blue moon since 2012 – and the last until 2018. Astronomers say it will only occur 12 times between now and 2043.
Sadly it doesn't mean the moon will be blue – it's actually the unusual occurrence when a full moon appears twice in the same month, although definitions of a blue moon vary.
Occasionally the moon can appear to be blue but this is only when there is unusual conditions causing dust or smoke to distort the moon's colour.
Although that isn't expected this time around, today's blue moon does coincide with the start of the meteor observing season, which will peak in August when the Persieds meteor shower will be visible.