The seasonal definition of Blue Moons does not line-up with the monthly definition, meaning the last time Earth saw a seasonal Blue Moon was actually May last year.
Whenever Full Moons land on Halloween, they have to be a Blue Moon because of the 29.5-day-long lunar cycle.
And although Blue Moons arrive every two and a half years, Full Moons taking place on the scariest night of the year only occur every 19 years.
This is due to the fact the lunar calendar repeats itself every 19 years.
This cosmic fact, called the Metonic Cycle, was actually first discovered in 432 BC by Athenian astronomer Meton.
source: express.co.uk