If June 21 is the day when we receive the most sunshine, why is it regarded as the beginning of summer and not its peak? And similarly, why is December 21, the day of least sunshine, the beginning of winter and not mid-winter day?
Blame the oceans, which heat up and cool down only slowly. By June 21 they are still cool from the winter time, and that delays the peak heat by about a month and a half. Similarly, in December the water still holds warmth from the summer, and the coldest days are still (on the average--not always! ) a month and a half ahead.
And what about our distance from the Sun? It, too, varies, because the Earth's orbit around the Sun isn't an exact circle. We are closest to the Sun --- would you believe it? --- in the cold wintertime, around January 3-5. This may have an interesting implication for the origin of ice ages, as will be explained later. It also ties to an interesting story of the unusually bright Moon of December 22, 1999, see http://www.phy6.org/stargaze/Imoon3.htm.