There's the old saying, "once in a blue moon." It is a term used to describe something that happens very seldom. But it is also used to describe something that happens in the sky and this Saturday skywatchers will witness a Blue Moon.

A Blue Moon is either the second full moon in a month or the third full moon in a season that has four. Local skywatcher Ken McAllister of Steinbach says because we will have four full moons this spring, Saturday's is referred to as a Blue Moon.

In addition to that, Mars will also appear more red than normal. McAllister says in actuality, the colour of Mars will be no different Saturday than any other day. But, because it will appear in the sky right next to the full moon, it will seem to be extra red.

"Mars normally appears red but we don't necessarily notice it because it just looks like another star," explains McAllister. "But coming up when it's right near a full moon, it's more easy to notice."

McAllister says Saturn will be joining the party as well Saturday night. He says the moon, Mars and Saturn will make a triangle in the sky and be quite beautiful to watch.

The last Blue Moon was August 20, 2013, while the next one will happen January 31, 2018. McAllister says on average it happens once every nineteen months.