In the Northern Hemisphere, the year is divided into four seasons. Each season begins at a solstice or an equinox.
In the Southern Hemisphere, the dates (and the seasons) are reversed. The summer solstice (still the longest day of the year) falls around December 21, and the winter solstice is around June 21. So when it's summer in North America, it's winter in South America (and vice versa).
The spring equinox brings the start of spring, around March 21. At the equinox, day and night are of about equal length.
The summer solstice, which happens around June 21, has the longest daylight time. It's also the first day of summer.
Fall begins at the fall equinox, around September 21. Day and night are of about equal length.
The winter solstice, around December 21, has the shortest daylight time and officially kicks off winter.
Earth's Seasons: A Video
Watch this video to visualize how the tilt of Earth's axis causes seasons to change as it orbits the Sun. You will learn about the solstices and equinoxes.