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.

The spring equinox marks the start of astronomical spring. The date is determined by the Earth's tilt on its axis as it travels around the Sun. At the equinox, the Earth's axis is tilted neither towards nor away from the Sun, making day and night nearly equal in length. The equinox happens twice a year - in March and in September.