Southeast Texas will witness a somewhat rare occurrence tonight when a blue moon will appear; the last one until 2018. If the moon actually appears blue, it’s caused by a rare type of dust in the atmosphere. According to NASA, blue moons are very rarely blue. Most are pale gray and white.

Experts at NASA say a blue moon is defined as any time there are two full moons during a calendar month. While most years have 12 full moons, this year has 13. A truly blue colored moon can occur on rare occasions, according to NASA, with most being spotted after volcanic eruptions.

It’s also possible Friday’s moon could be red. “Often, when the moon is hanging low, it looks red for the same reason that sunsets are red, NASA explains. “The atmosphere is full of aerosols much smaller than the ones injected by volcanoes. These aerosols scatter blue light, while leaving the red behind.”

The moon will be visible at sunset, and NASA encourages those who see it to go ahead and celebrate by doing something you only do “once in a blue moon.” You do have an excuse, after all.