On October 31st there will be a blue moon.
But what is a blue moon? A blue moon is, by some definitions, a second full moon in a month of the calendar.
The first full moon this month was on October 1st. The next full moon is on Halloween.
A blue moon is not blue, although a visually Blue Moon (the Moon appearing with a bluish tinge) may occur under certain atmospheric conditions.
After this month, the next time there will be a blue moon is on August 1 and 31, 2023.