When Is the Summer Solstice 2021 and How Is the First Day of Summer Celebrated? Earth's tilted rotation brings a shift of seasons and the longest day of the year north of the equator The summer solstice marks the first day of summer in the Northern Hemisphere, bringing the longest day of the year, the shortest night and, in the northernmost Arctic, a midnight sun that shines around the clock. It coincides with the winter solstice in the Southern Hemisphere, which marks the year's shortest day and the beginning of winter there. "The solstice is an excellent timekeeping moment, for figuring out where you are in the cyclical nature of the planet," said astrophysicist Jackie Faherty at the American Museum of Natural History in New York. Solstices occur every June and December. The term "solstice" comes from the Latin words "sol," which means sun, and "sistere" which m