2020 may not have been the best year to travel (or leave home), but it was the best year for celestial events. This spring we saw a "worm full moon" in March followed by a pink supermoon in April; in July we saw a double meteor shower, a lunar eclipse, and numerous other astronomical phenomena; in August we can expect to see the Perseid meteor shower and the appearance of Venus coinciding with the crescent moon; and in November we can expect to see a "blue moon" in the sky.

But as you may recall from this list of 2020 full moons, there will also be a blue moon in the sky this year. This rare phenomenon will occur on October 31, 2020. Yes, a blue moon on Halloween night!

Now, in a perfect world, a blue moon would mean that the moon appears blue at midnight. Unfortunately, this is not usually the case. The moon is considered a "blue moon" not because of its color, but rather because of when it happens.

As you may already know, a full moon usually occurs once a month; if a full moon occurs 13 times a year instead of 12, it means it is a blue moon (its unusual nature is the basis for the term "once in a blue moon").

There are two types of blue moons: according to Earth Sky (opens in new tab), there are two types of full moons: the second of two full moons in one calendar month (aka monthly blue moon) and the third of four full moons in one season (aka seasonal blue moon ) are considered blue moons.

It is likely that the October blue moon will not actually appear blue to us on Earth, but it is not entirely impossible: according to timeanddate, the moon will appear blue if the atmosphere is filled with dust and smoke particles of a certain size. Thus, if a major volcanic eruption were to occur, the moon could appear blue: the eruption of Mount St. Helens in the United States in 1980, the eruption of Mount Krakatoa in Indonesia in 1983, and the eruption of Mount Pinatubo in the Philippines in 1991 all made the moon appear blue.

This year's blue moons will be monthly blue moons, with full moons on October 1 and October 31, 2020; since there are two full moons scheduled for October, the latter is called a blue moon.

According to meteorologist Brian Rada of AccuWeather (open in new tab), we won't see another blue moon on Halloween night until 2039. With that in mind, it's best to mark your calendars now!