When people say something occurs just once in a blue moon, they mean it’s very rare. These days, the speaker is often referring to the modern folklore that, whenever there are two full moons in a calendar month, the second one should be called “a Blue Moon.” Going by this definition, the United States will experience a Blue Moon on the evening of August 31. The first full moon of the month already happened on August 1.

Most months have only one full moon. The 29.5-day cadence of the lunar cycle matches up almost perfectly with the 28 to 31-day length of calendar months. Indeed, the word “month” comes from “moon.”

Occasionally, however, the one-to-one correspondence breaks down when two full moons squeeze into a single month. This happens on average about once every 2.5 years, which…. is actually not all that rare, really.

Will the moon truly appear blue in color tomorrow? Smoke from volcanoes and forest fires can cause a change to blue from the usual pale gray, but it’s unlikely, despite all of the wildfires burning in the hot, dry United States this month. The amount of smoke to create the effect has to be unusually massive. Some kinds of blue moons are rarer than others, it turns out.

Learn more and watch a video about the facts and myths of blue moons from NASA.