A rare cosmic occurrence will take place in the sky on July 31. The date marks the second full moon to appear in the month of July. This phenomenon, called a "blue moon," happens but once every three years, according to CNN.

The cosmic event doesn't exactly make the moon turn blue, contrary to popular notion. But the occurrence is still pretty special because rarely do months get two full moons. Hence the term, "once in a blue moon."

There are usually 12 full moons in a calendar year, one for each month. But because the lunar calendar and the monthly calendar aren't always in sync, sometimes we are treated to full moons twice in one month.

Such is the case for July, when the first full moon appeared last July 2, and then once again this July 31.

On Friday, the blue moon will be seen at 6:43 a.m. EDT, according to Space.Com. The next time this rare occurrence happens will be in January 2018.

Over the years, the definition of the term "blue moon" has changed, but it is the 1946 definition, to allude to the second full moon in a month, that modern-day scientists adhere to today, according to another Space article.

Meanwhile, the color of the moon does turn a bluish speck whenever there is dust and smoke in its atmosphere, said astronomer Larry Ciupik, via NBC.  If this happens on Friday, then we're likely to see the moon turn a bit blue, but Cuipik said he's not counting on it. The moon that day may not look odd and may appear as any other full moon.

But the allure of the blue moon lives because of its cosmic mystery. "It's also a kind of romantic thing. I think it's kind of primal. You look at the moon, and it's kind of primal. You could be seeing this a thousand years ago, the same thing," Cuipik told NBC.