Spring is a season of nature reawakening from winter's long dormancy. By the modern calendar, it begins with the vernal equinox (about March 20). This is the day when the sun's vertical rays cross the equator, and daylight / night time hours around the world are theoretically about equal. Daylight hours continue to increase by about three to five minutes a day, reaching a peak of daylight with the summer solstice of about June 21.
It is not coincidental that Easter and the equinox are closely associated. Keeping the vernal equinox stable around March 20 on the calendar was a major goal of Gregorian reform of the calendar in the 16th century. For Christians, the celebration of Easter, between March 22 and April 25, is an affirmation that spring is finally here. Others would give that distinction to St. Patrick's Day of March 17. This is the largely secularized Saints Day where green symbolizes the shamrock, Ireland, and spring,often accompanied by parades, music, and frequently, by over indulgence in green beer.