The vernal equinox - when winter changes into spring - will occur at 1:32 p.m. EDT Saturday.
At that moment, the sun will cross precisely over the Earth's equator. There will be equal amounts of day and night on this day.
The days will then grow longer each day, reaching their limit on the summer solstice, June 21. The days will then grow shorter, until the autumnal equinox on Sept. 22, when there will again be equal parts of day and night.
Equinoxes have been recognized for thousands of years. Easter is always on the first Sunday after the first full moon after the spring equinox.