A rare blue moon is set to light up the night sky in Toronto this weekend, so be prepared to admire the celestial event in the early morning hours of Aug. 22. 

But despite what its name suggests, the moon won't actually appear to be blue this weekend. Instead, it'll look like most other full moons we're accustomed to seeing, though it will still be rarer than most. 

There are actually two types of blue moons—seasonal and monthly—and neither is something we get to see on a regular basis.

The particular moon that will grace Toronto's skies beginning late on Saturday night is called a seasonal blue moon because it's the third full moon in a season containing four full moons rather than three—a phenomenon that only happens once every two or three years. 

A monthly blue moon, on the other hand, refers to the second full moon in a calendar month with two full moons rather than one—a celestial event that also comes around every few years and last happened on Halloween of 2020.

Our last #BlueMoon came on the night of Halloween 2020. But that full moon - like most Blue Moons - was blue in name only. This week, we'll have another sort of Blue Moon, coming up on August 21-22: https://t.co/jXRFupDeOf 🌚

📸 Priya Kumar in Oman, 2012. Thank you, Priya! pic.twitter.com/oAzB7dtRUs

— EarthSky (@earthskyscience) August 17, 2021

Though both events only occur every once in a while, a seasonal blue moon is just slightly more rare than a monthly one, making Sunday's celestial event all the more exciting.

The next time we're expected to see a rare full moon such as this one is Aug. 19, 2024.

So be sure to keep an eye on the sky in the late hours of Saturday and the early hours of Sunday this weekend, because you know what they say about rare celestial events: they only come around once in a blue moon.