I think it's September 24th-25th, but the flyer they passed out on board the train is in my car at home. If somebody who knows more doesn't respond, I'll dig it up when I get home and update this if needed. The D&S website doesn't have the details posted yet. The full thing is 2 days, with an overnight in Silverton (lodging not included in price, but they'll hook you up, as I remember), or you can go for a day and they'll bus you to/from Durango.
Also, Railfest is a month earlier, August 24-28.
Nathan