Depends on your ticket type. If you have an annual pass, the only option is buying day of.
If you bought tickets not in a package through Disney, I believe you may have to purchase daily as well, but you may be able to call ahead of time and get it added for your entire trip.
If you bought tickets as part of a package, you should be able to update your package or you can add day of.
If you are purchasing in advance, you can only do it for the entire trip. If you want only select days, you'll have to purchase day of.
Daily purchases are done in
mde in the Genie section.
Also all ILL$ are day of purchases only.