This company and this company, and I'm sure there are others, will take you to major chateaux from Tours 7 days a week. I just googled them, can't vouch for them. Weekend will be busier than weekdays. Nothing in tourism is dead on Sunday! Weekend in Paris will have less traffic than weekdays, but you're pedestrians anyway. Pick your days of the week based on your convenience.
But keep in mind that "two days one night" really means an afternoon and a morning if this is a side trip from Paris. You'll see more and have an easier time if you can spend two nights in Tours or another Loire base (Tours is probably best without a car).