I'm the same with weekends. My very best days have been weekends, usually Sundays, but I've also had plenty where the phone has been on 11am to 11pm and not rung once.
I don't worry about whether I should work a certain day or not - if there's something I want to do with friends or family then I take the day off, and because most of them work Mon to Fri, stuff is often arranged for the weekend.
I could drive myself insane trying to predict which days will be busy and which days slow!