http://www.sfgate.com/cgi-bin/article.cgi?f=/n/a/2011/01/02/financial/f103749S31.DTL#ixzz19wKI4bEg I remember a heatmap which was posted here which I think showed similar results as well.
I think this is some normal "Seasonality" at work Don't tell anyone though, or it will stop working... marc