Seasons

Season is a period of the year marked by special events and activities that involves changes in weather and amount of daylight. Seasons result from Earth's orbit around the Sun and Earth's axial tilt relative to the ecliptic plane. In temperate regions, four calendar-based seasons are generally recognized: spring, summer, autumn and winter. For tropical regions, we have two seasons: the rainy season and the dry season. Some have a third cool, mild, or harmattan season, present around sub-sahara region. Regardless of the time of year, the northern and southern hemispheres always experience opposite seasons. This is because during summer or winter, one part of the planet is more directly exposed to the rays of the Sun than the other, and this exposure alternates as the Earth revolves in its orbit. For approximately half of the year (from around March 20 to around September 22), the northern hemisphere tips toward the Sun, with the maximum amount occurring on about June 21. For the other half of the year, the same happens, but in the southern hemisphere instead of the northern, with the maximum around December 21. The two instants when the Sun is directly overhead at the Equator are the equinoxes. Also at that moment, both the North Pole and the South Pole of the Earth are just on the terminator, and hence day and night are equally divided between the northern and southern hemispheres.
Excerpt from: Online sources