Thursday is the first day of spring, or the vernal equinox, in the Northern Hemisphere. During an equinox, the center of the sun is on the same plane as the equator. After the vernal equinox in March, the Northern Hemisphere tilts toward the sun until the June solstice, when the sun begins to migrate back south.

An equinox is a phenomenon that occurs twice a year — once during the spring equinox and once during the autumn equinox. In the northern hemisphere, the spring equinox typically falls around March 20, signalling the end of winter and the arrival of spring.