Today the Sun reaches its northernmost point in planet Earth’s sky. It is a widespread misconception that summer “officially” starts on the day of the summer solstice, June 21 or 22, which is the longest day of the year. Many believe that there is some valid scientific reason for doing it that way. However, no scientific or governmental body has ever formally declared that summer starts on the solstice.
Certainly there is no good scientific reason for doing so. In the Northern Hemisphere the period of maximum daylight falls roughly between May 7 and August 7–in other words, the six weeks before and after the solstice. The period of maximum temperature, on the other hand, is June 4 through September 3. (The period of max temperature in the mid-latitudes always lags about 25 to 30 days behind the period of max daylight, due to the fact that the earth heats up and cools off relatively slowly.)
Here is what the Bad Astronomer has to say about the beginning of season.