We all know the Winter Solstice is the shortest day of the year, but is it really? The Winter Solstice is when the earth’s axis is tilted the farthest away from the sun. The actual solstice occurs just for a moment. For 2010 the Winter Solstice is occurring on December 21st at 23:38 UTC. so for those of us on the east coast of the US it is occurring at 18:38ET. So the shortest day for us is December 21st. However if you lived in Frankfurt Germany, the winter solstice occurs on December 22 at 00:38 so at shortest day is actually December 22nd. For Germany the 22nd is shorter than the 21st by less than 1 second.
Did you also know that the winter solstice changes each year? The date actually flips between December 21st and the 22nd. This change is due to the irregular rotation of the earth. Next year the winter solstice occurs on December 22nd at 05:30UTC. So in 2011, if you live on the west coast of the US, your shortest day will be December 21st, even though the winter solstice is on the 22nd. On the west coast the exact time the winter solstice will occur is December 21, 2011 at 21:30PT.
So when is the shortest day of the year? It depends upon where in the world you are when the Winter Solstice occurs.