Don't forget to look up at the sky on Sunday night. A full blue moon, which The Weather Network calls "the rarest full moon of 2021," will be visible in Montreal.

This full blue moon is rare because it's the first one to happen in the summer in five years and, according to The Weather Network, there won't be another one like it until 2024.

So, what even is a blue moon?

The Weather Network's traditional definition is "the third Full Moon in a season with four Full Moons," which is what we will see on August 22.

While seasons typically have three full moons, some seasons, like summer 2021, have four: June 24, July 24, August 22 and September 20.

The moon won't actually appear blue — that's just the name used to describe its rarity since the moon doesn't often appear blue.

When we see the moon change colour, The Weather Network says it's usually orange or red due to a lunar eclipse or when there are smoke and ash particles in the air.

According to NASA, the moon can look blue when the air has lots of "particles slightly wider than the wavelength of red light (0.7 micron)--and no other sizes present. This is rare, but volcanoes sometimes spit out such clouds, as do forest fires."

Regardless of its colour, this weekend's full blue moon is going to be beautiful so don't miss it!

This article's cover image was used for illustrative purposes only.