The saying "once in a blue moon" is especially pertinent this week: This Sunday (Aug. 22), the full Sturgeon Moon is expected to impress skygazers, particularly because of its "blue" designation.

Typically, the term "Blue Moon" refers to the second full moon within the same month. The last one rose on Oct. 31, 2020, when an eerie Blue Moon lit up the night sky on Halloween. But there's a lesser-known definition, dating to 1528, which applies to the third full moon in a season with four full moons, according to NASA.

In general, each season has three full moons. But summer 2021, which began June 20 and ends Sept. 22, has four full moons (June 24, July 23, Aug. 22 and Sept. 20). Seasonal Blue Moons are uncommon, occurring about once every two to three years, according to EarthSky. The last one rose on May 18, 2019, and the next one won't shine until Aug. 19, 2024.