The start date is May 5th, the 'official' beginning of the 50th Anniversary Celebration.
I just made a rez for September and they offered it to me, but I didn't have my airline yet, so I declined for the moment. I'm still not 100% sure if I want to try it. Even though it's free, I've never been a huge fan of Mear's and I'm curious to see how long it takes you to actually get to your hotel, as there are usually many stops at the various hotels.
I'm thinking I'll stick with the TownCar.