Interesting question. I often work at the weekend but that's to fit stuff in, so I don't charge an uplift to my normal rate. However if they've specifically booked you for a weekend day I don't know... I'd say it's down to the relationship you have with them - if they think working on a Saturday is a big deal, charge a bit more. If they think it's just run of the mill then I suspect you'll have a fight on your hands to get the rate up.