Another name for the first day of spring is the vernal equinox. "Vernal" comes from the Latin word for "spring," and "equinox" comes from a Latin word meaning "time of equal days and nights."
During an equinox, the sun passes directly over the equator. This happens in March and September of each year. In the northern hemisphere, the vernal equinox occurs in March and heralds the arrival of spring. At this time, the day and the night are of nearly the same length. The vernal equinox also signals the time during which the Earth's axis begins tilting toward the sun so that days are longer and the temperature is warmer.