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

The summer solstice, the longest day of the year in the Northern Hemisphere, falls on June 20 this year in North America, and June 21 in Europe.

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.