How often does the Earth get a “shortest day of the year?”
While the northern hemisphere experiences (and celebrates) the shortest day of the year, (Winter Solstice), the southern half of the planet enjoys their longest day, (Summer Solstice). The reverse likewise holds true. Except at the equator where the days and nights are always 12 hours each.
The cause is Earth’s 23.5 degree axial tilt. This tilt is also responsible for the shift of the positions of sunrise and sunset over the course of the year. At most terrestrial locations, the sun does not rise due east and set due west except twice a year (Autumnal and Vernal Equinoxes).
Today is 26 December 2013 and our northern hemisphere days are growing longer and our nights shorter. But don’t look for any real warming trends for at least another two or three months. It takes time for the planetary surface to react to these changes in a significant way.