On Halloween night we will be treated with a blue moon! Two full moons in calendar month happens only once every 2½ - 3 years on average, according to NASA's National Space Science Data Center.

Every month has a full moon, but because the lunar cycle and the calendar aren't perfectly in line with each other, about every 2.7 years we wind up with two in the same calendar month. Full moons are separated by 29 days, while most months are 30 or 31 days long; so it is possible to fit two full moons in a single month.

A full Moon on Halloween occurs roughly once every 19 years. But this will be the first time a Halloween full moon has appeared for all time zones since 1944, according to Farmers' Almanac.

There hasn’t been a Blue Moon on Halloween since 1974 and there won’t be another one until 2039.

Sometimes, like in 2018, a ‘Blue Moon’ occurs twice in a year. The time span between two full moons is about 29.5 days, so there is never a blue moon in February.

After Halloween, the next time we see two full moons in a month won’t be until August, 2023.

If you miss this Halloween's full moon, there are still 4 more chances this century: 2039, 2058, 2077, and 2096.

In the 1100 years between 1550 and 2650, there will be a total of 456 monthly Blue Moons.

If you truly want a spooky moon on Halloween, look for  a “Black Moon" -- when there's a second New Moon in a calendar month. The next time there's a Black Moon on Halloween won't be until the year 2035 and it hasn't occurred since 1997.

To read more about how the term 'Blue Moon' was first coined, click HERE.