In 2017 summer will begin on Wednesday 21st of June at 04:24 UTC. This season will last 94 days and it will end on 22nd 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).