I think most of us were brought up with "June 21st - longest day and December 21st - shortest day". Doing a quick search on the net, it seems that it works on a 2 year cycle; last year and this year it is Dec 22 and on the 21st in 2008 & 2009. Some years the difference in daylight is as much as - wait for this - one second! December 21st is the start of the winter solstice so I will always say that the shortest day is the 21st. Let's not quibble over one second - unless it is in a time trial!
We will start to see the benefit by the end of January, I am also happy to get past the shortest day. But also get bloody depressed on the longest day knowing that we are once again turning towards winter.
Never used to pay any attention to it until I became a cyclist.