The change in seasons is caused by the varying angle of the Earth's tilt from one part of the year to another. A common misconception is that the change in seasons is due to the Earth's slightly elliptical orbit bringing the planet closer to the sun in summer, but, as About.com notes, seasons are different in the northern hemisphere and the southern, which are the same distance from the sun.
The shortest day of the year is called the winter solstice. The winter solstice occurs on the 21st of December in the Northern Hemisphere.
The longest day of the year occurs on the summer solstice, which takes place in June in the Northern Hemisphere and December in the Southern Hemisphere. The solstice occurs between June 20 and 22 or between December 20 and 23 each year.