You should call asap and determine if the house/account is serviceable. With most companies, the calendar is available 2 to 3 weeks out. If you are moving to brand new home, it has to be listed in Comcast data base before you can order service. With a current residence/account, the vacating owner may not have put in a termination/transfer of services, or the account could be delinquent.
Some promotions are very limited, such as a 3 day blitz. When the promo time ends, the billing codes are removed from the system. When you see a deal, call.