Following the Mid-Autumn Festival on 1st October, a full moon will occur again on the night of Halloween on 31st October! When there are two full moons in a single calendar month, the second one will be called the Blue Moon.

The meaning of the Blue Moon in astronomy was said to be originated from a book published in 1937. In 1946, an article in the magazine Sky & Telescope mistakenly defined the Blue Moon as the second full moon in a calendar month. As this magazine was popular, this misdefinition was widely spread and gradually become the current public understanding of Blue Moon. Blue Moon does not occur very frequently, but yet it doesn't mean our moon looks blue on the day.

There is an idiom "Once in a Blue Moon" to describe something that happens rarely. However, the appearance of the Blue Moon is actually not particularly uncommon. The lunar cycle lasts 29.5 days, so there will always be a full moon in each calendar month, which has 30 or 31 days, except for February. Therefore, as long as the first full moon occurs at the beginning of a month, the second one may appear again at the end of the same month. On average, a Blue Moon will appear every 30 months and the next one will come on 31st August, 2023.

But having that said, it will take 19 years to have the next full moon appearing on the Halloween night. If you don't want to miss the opportunity to see the Blue Moon on Halloween night, you might go outside or look out the window to find the round and big "Blue Moon" in bright white with the naked eye!