Two things:
1. You get a number of "credit days" equal to your room nights.
2. You don't have to use your credits on a specific day.
So you may have unused credits at the end of your stay, which, as jillgunter said, you can use on your last day, even after you check out. (From the
DDP brochure: "All unused meals and snacks expire at midnight on your package reservation checkout date.")
It depends partly on when you arrive at WDW and when you leave. You can think of one day's worth of credits as belonging to your first day or your last day, or split them between the two, depending on how many mealtimes you'll be there for on those days. Again, your credits aren't tied to specific days; you can use more of them on Tuesday, say, and make up for it by using fewer on Wednesday or Thursday. People do this a lot with the higher-end plans: they spend 2 TS credits on a signature meal one day and go without a TS meal on another day.
We don't use any of the DDPs, as they don't fit our eating style. They're inflexible in that you can't substitute a soup or salad for a dessert. But I do admire their flexibility in terms of credits vs. days.