To me, summer officially starts when I have gone to the beach, saw chicks in bikinis and went swimming in the lake. So for me summer started two weekends ago :) The seasons are slowly changing, don't listen to the calendar :)
Summer officially starts on June 21 of each and every year and comes to an end on September 22 in the Northern Hemisphere, especially in the US. It extends for the entire months of December through February in the Southern Hemisphere. Check out calendar-updates.com/info/holidays/us/summer.aspx for more information.