What is a Blue moon?
First of all, for the record, the Moon will definitely not take on a blue hue on Saturday, October 31.
A Blue Moon is instead a fascinating phenomenon that takes place because Full Moons actually occur exactly every 29.53 days.
This means – from an astronomical perspective – a lunar ‘year’ of 12 cycles lasts a little over 354 days.
As a result, this means the lunar year comes almost 11 days shorter than the time it takes the Earth to orbit our Sun, meaning some years occasionally enjoy an extra Full Moon.
source: express.co.uk