Statistically, it is harder to reach Repubs at home in order to poll them on the weekends. Sunday is a no-brainer because Repubs are more likely to be church-goers. As to Fri evening and Saturdays, my only guess is that Dims stay home 'cause they are unemployed and don't have any money .. ;-) Thus polls trend higher on weekends for sKerry and Bush recovers during the week.
:-} Yeah, I know the theories but I've never seen hard data to support them. But I have seen the theory morph into a law this election cycle. Its like clockwork.