From Stone Age astronomy to Viking law and Roman goddesses, cultures across time have honored the longest day of the year

People have been celebrating the summer solstice with elaborate rituals since prehistoric times. But humans aren’t the only species to take mark June 21 as a special time. Studies are showing the summer solstice is an important cue for plants too.

The 2025 summer solstice falls on Friday, June 20, at 10:42 p.m. ET, according to NASA and the Old Farmer's Almanac. This marks the longest day of the year in the northern hemisphere, when the Earth's tilt positions it closest to the Sun.