The seasons result from the Earth's axis being tilted to its orbital plane; it deviates by an angle of approximately 23.5 degrees. Thus, at any given time during the summer or winter, one part of the planet is more directly exposed to the rays of the Sun. This exposure alternates as the Earth revolves in its orbit. At any given time, regardless of season, the northern and southern hemispheres experience opposite seasons.
21 March is the Vernal Equinox and represents the beginning of spring in the northern hemisphere. It is the point at which the Sun crosses the Equator as it travels northwards. At this time day and night are of equal length, hence equinox - 'equal night'.
21 June is the Summer Solstice or Midsummer's Day in the northern hemisphere, when the Sun has reached its further point north of the Equator (Tropic of Cancer). As more sunlight reaches the northern hemisphere the days last longer than the nights. The day after, the sun begins to travel south and the days start to shorten.
21 September is the Autumnal Equinox when the sun crosses the Equator going south.
21 December is the Winter Solstice and is shortest day in the northern hemisphere; it is the middle of summer in the southern hemisphere. The Sun has reached its furthest point of travel south - the Tropic of Capricorn.