Do you get credits on the dining plan for both check in and check out day... ???
You get credits per night of your stay.
The credits expire at 23:59 on your checkout day, so you can use them even after checking out of your resort.
So you won't have enough for a full day's worth of meals on both check in and check out day.
Essentially you'll have one day of credits to split between check-in and check-out day (unless you decide to use more during your stay). So if you check in in the AM, you could have lunch and dinner, and then have breakfast on check out day. If you check in in the afternoon, you'll have dinner that day, and breakfast and lunch on check-out day.