In 2015 summer will begin on Sunday 21st of June at 5:38pm UTC. This season will last 93 days and 15 hours, and it will end on 23rd of September with the beginning of autumn.
The beginning of seasons is conventionally established by particular positions of the Earth during its orbit around the Sun. So the summer solstice occurs when the Sun reaches its greatest height, i.e. the declination is +23º 27′ for the northern hemisphere. In the southern hemisphere winter begins.
Solstice is derived from the latin words sol (sun) and sistere (to stand still).