To answer the OPs question, it is really up to you. You can do a post-dated cancellation and that would suit your needs. Once the installation happens, there is not really going to be a way to enjoy it (cables will likely no longer be connected anyway). You could set it for the 9th, being that the installation is not scheduled until the 8th. At any time, you can change the date if needed (installation does not happen or there is an issue).
Hate to hear that you are leaving DISH, but when something you don't have and really want is the reason, I cannot say I blame you. Hopefully in the future you allow us to provide service again.