Spring, as marked by the vernal equinox, begins in March in the Northern Hemisphere. In the Southern Hemisphere, or below the equator, spring begins in September.
Because Earth tilts on its axis, there are periods of time where the upper and lower halves of the planet are relatively closer to the sun. The vernal equinox, also called the spring equinox in the Northern Hemisphere, marks the time each year when day and night hours are equal. After this point, the relative amount of sunlight received by the Northern Hemisphere increases until the summer solstice in June, which is the longest day of the year.