Patty,
Sunday by far is the better day to check in since the weekenders (in on Friday and out on Sunday) leave which frees up more sites. In the spring and fall especially the weekends book up well in advance and make it harder for folks to get extended vacations closer to arrival that include that elusive weekend.
While what John says is true that all sites are pre-assigned before arrival, you can try negotiation as I have. You might have requested a specific loop but been pre-assigned a different loop. In your best Sunday manners voice, ask if they can move you to the desired loop. That person will get swapped to another site (your old one?) and will be none the wiser.
Also arriving earlier in the day helps your chances. While the official check-out time is 11 am and check-in is 1pm, I've arrived early and wanted loop 1500 (Partial) but was assigned 2000. I told the CM I was willing to wait if I could get 1500 and someone who was leaving that day hadn't had their site cleaned and prepped yet. So the CM agreed but wouldn't tell us the site number. We did all the check-in, went off-property to lunch, and while eating got a text telling us our site number and that we could drive right down to it after eating and set up.
Getting closer to Christmas, that "weekender" impact gets muddled as the new arrivers have longer reservations (like you do). But given the date of your arrival and the length of your reservation, I understand what you're doing and a late Sunday morning arrival gives you the best chance.
Bama Ed
PS - no guarantees of course, but every little thing you do hopefully adds up to enough...
