I believe I have answered this question before. Look at some of the other questions in the section under Sun. the answer has to do with the observer's latitude on the Earth, the speed of the Earth in its orbit around the Sun, and the inclination of the axis of the Earth, and its orbital plane, relative to that of the solar system. If the Earth orbited in a circular orbit with constant speed, in the plane of the solar system, and with its axis tilted 90 degrees to the plane of the solar system, these dates would coincide with the Winter Solstice.