Get ready for a rare sight: the legendary “blue moon” will be visible in the sky late tonight and into early Friday morning.

Of course, you won’t see an actual blue moon — it just means that the moon you are about to see can only occur in very specific conditions that are extremely rare to behold, according to various media reports.

It’s the timing of the moon that is so rare, not its color, which will be no different than usual. Full moons typically happy just once a month, and a blue moon is a colloquial term that refers to a second full moon in the month. That’s what in store — not quite a mind-blowing experience, but certainly a rare sight for astronomy and star gazing buffs.

It happens ever two or three years, and is the originator of the phrase “once in a blue moon.” This month, the first full moon happened on the very first day — July 1 — while the blue moon will take place July 31. It’s the first time this has happened since all the way back in August 2012.

This full moon will be the most full at 12:42 a.m. on July 31.

There’s also the phenomenon known as the “supermoon.” This is different from a full moon in the sense that it happens when the moon is at its closest point to Earth, making it appear massive and much brighter than it usually does. A blue moon is merely an oddity of the calendar.

The next blue moon is expected to happen on January 2018, so catch a glimpse of it while you can.