I echo what Lynda said above. On the tour you will walk around the town and learn about the history of Chartres, as well as get a tour of the Cathedral. That takes you up to about noon or 1pm on Monday. However most shops in town are closed Sunday and Monday. I suggest you plan ahead to leave town on Monday afternoon; check the train schedules and figure out some place to go for the rest of the day and perhaps dinner too (I think that is one of the nites when dinner is on your own). Rambouillet looks to be nearby, Le Mans is about an hour by train, etc. If you want to see the shops in Chartres then you will need to get there on Sat (the 1.5 days early you asked about) but plan on leaving town on Sun as well.
This is a nice tour, but there a couple of days where it is going to be up to you to either plan ahead to explore on your own, or if not comfortable with that then bring a good book to read. Another such day is the market day in Sarlat. It is fantastic, more booths than you can imagine! However there just isn't a lot else to do in Sarlat, and after a couple of hours you have pretty much covered every nook and cranny of the market. I would suggest again looking at rail options for mid-morning on, or, even better, rent a car for the day to go see more of the Dordogne region. This takes some advance planning, and I know a lot of people turn to a tour because you leave the planning up to them, so I'll repeat the advice about not running out of books to read!