In the Northern Hemisphere, spring covers March, April and May; summer covers June, July and August; fall covers September, October and November; and winter covers December, January and February. There are no official allocations of the four seasons, though, and in the Southern Hemisphere, the seasons are reversed. When it's summer in the Northern Hemisphere, it is winter in the Southern Hemisphere, and when it is spring in the Northern Hemisphere, it is fall in the Southern Hemisphere. Many countries south of the equator, however, tend to have more tropical climates in which the evolution of the seasons is less evident.
The seasons are intended to coincide with the equinoxes and solstices that occur with the sun. A solstice is when the Earth's axis is most tilted toward and away from the sun. During the summer when the North Pole leans toward the sun, days are longer. In the winter, when the North Pole leans away from the sun, days are shorter. The solstices are in March and December. During equinoxes, the sun is directly over the equator, creating days and nights of mostly equal length. The equinoxes are in March and September. The dates of the solstices and equinoxes are rolling, however, whereas the dates of the seasons are fixed, so there is not always a correlation.