As the PP said, the magic bands come with the hotel reservation, and not the ticket. Assuming you are a US resident, and customize the bands more than 30 days before your trip, the bands should arrive at your house 10-14 days (if I recall correctly) before your hotel stay. So unless you are going to be leaving your house earlier than that, you should get them before your off property stay. They should work as soon as you receive them. If buy your tickets from a reputable 3rd party reseller such as Undercover Tourist, you can buy tickets for the duration of your entire vacation, use them with the Magic Bands, and most likely save a few bucks over buying the tickets direct from Disney. (Other resellers may or may not be cheaper than UT, but make sure they are reputable)
If you are not a US resident, or don't customize the bands at least 30 days in advance, they will be waiting at your hotel for your arrival.