Summer gets its official start in the Northern Hemisphere with the arrival of the summer solstice. The sun will trace its highest, longest arc across the sky Friday, making for the longest day and shortest night 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.