Well, you have to consider families and lone mothers for example. They need the day off when their children are not at school, or they have to invest their salary in a baby sitter and they won't earn anything, as they don't get payed well anyway. And of course you could say that there are enough people without a family that could work at these days, but first of all companies don't work that way, and secondly, these people's friends are off at Sundays too, so they also don't want to work then.
Sorry, but this discussion is a big thing in Switzerland, I just wrote down the "against"-arguments that came into my mind first ;-)
Well, I'm heading out, have a nice time everybody.