Seasons start and finish with the equinoxes and the solstices. For example, summer begins with the summer solstice, which occurs in the northern hemisphere at around June 21. During the summer solstice, the hours of daylight are greater than the hours of darkness.
Summer then ends with the fall equinox, which happens around September 21 in the northern hemisphere. During this equinox, the hours of daylight and darkness are equal. Fall in turn ends with the winter solstice, which happens around December 21, when the hours of daylight are less than the hours of darkness. Winter then ends with the spring equinox around March 21, when the hours of day and night are equal.