When you purchase a wall, you are only billed for the actual days of your event, you are not billed for the days leading up to the event and your wall does not go live instantly (unless you are already within 72-hours of your event).
If you're purchasing the Tweetwall in advance of your event, your wall will also include a free 72-hour pre-event monitoring window for testing and to ensure that your Tweetwall is not empty at the start of the event. If you purchase it more than 72-hours in advance, it'll go live 72 hours prior to your event starting, and continue until the event is scheduled to end. If you purchase it during that 72 hour window, it'll go live immediately after purchase and remain live until the event is scheduled to end.