To get a dining plan (unless you are an annual passholder, DVC member or eligible for the military room discount) you must purchase AT LEAST one-day base tickets as part of a package. It does not matter what length of ticket you purchase (for the free dining promotion it must be at least 2-day base tickets). You should note, however, that you are allocated dining credits based on how many NIGHTS you stay, not how many days. If you are on the regular dining plan and you stay 5 nights and 6 days, then you get 5 TS, 5 QS and 5 snacks per person.
You can order only as many meals at one time as you have persons listed on your key card. Other than that you can use your credits for meals in any combination and on whatever day you wish.