Spring starts when the vernal equinox occurs, which is usually on or around March 21. An equinox is a date when night and day are exactly the same length. Two occur during the year, one signaling the beginning of spring and the second signaling the beginning of autumn.
Weather forecasters do not use the vernal equinox to define the start of spring. They use the period in which temperatures begin to rise. This three-month period includes March, April and May. During this period rainfall increases, buds grow and deciduous trees regain their leaves. Animals also end their winter hibernations and enter a period in which many births take place.