When does summer start? Monday, June 1, marks the first day of meteorological summer, but when does summer officially begin? If you look at a 2015 calendar, you might spot that summer begins on June 21st this year. This uses the astronomical seasons, based on the solstices and equinoxes that occur through the year. Seasons occur due to the fact that the Earth is tilted from its axis by 23.5 degrees.