The summer solstice occurs this week across the northern hemisphere, marking the longest day of the year and the official start of astronomical summer. What You Need To Know Astronomical summer begins June 21 Climatologists and meteorologists break down seasons into 3-month cycles Keep safety tips in mind during the hottest time of the year June 1 is always the beginning of the meteorological summer. However, the official start of summer, astronomical summer, varies from year to year.