Summer officially starts at 10:51 UT (11:51 BST) on 21st June in the northern hemisphere with the northern solstice.
The solstice occurs when the northern hemisphere of the Earth is inclined at a maximum angle (23° 26′) towards the Sun and results in the longest period of daylight. In fact, for residents within the Arctic Circle, 24 hours of sunshine occurs and for those on the Tropic of Cancer, the Sun will be directly overhead. After the summer solstice, the daytimes shorten and the nights become darker and longer (something astronomers look forward to).
The opposite occurs in the southern hemisphere that sees the winter solstice at this same time. The reverse of this event occurs with the southern solstice on Dec 21st 2014 at 23:03 UT.