If you plan to go on Christmas Day, I'd highly suggest getting to MK early. We were in the exact same spot as you last year. Platinum AP holders, staying off site. I honestly don't remember what time we got to MK, but I'm going to guess it was somewhere around 8:30am. Took us a few extra minutes to go through the parking lot entrance, but not terrible. They had trams running back to back to back, so despite the massive flood of people coming in, didn't take us long to get to TTC. From there, CMs were directing us where to go. They put us on a bus. I don't think we waited more than 5 minutes to get on, so again great job on WDW part. In all honesty, the first few hours weren't all that bad. Crowded, sure, but still manageable. We had FP+ already booked, so that went smoothly. We rode a few rides that didn't need FP+, like Stitch, Laugh Floor, etc... I think we waited maybe 15 minutes for each of them. Did a little wandering around just to see the sights and take it in. Ate lunch...we pack our own food and amazingly enough we found a table at Tommorowland Terrace with no problem. After we got up from eating lunch, we noticed how incredibly packed it was. Main Street was just a sea of humanity.
MDE showed most rides had very long waits. It was just incredibly crowded. At that point, we decided we'd done what we wanted to do and left (which was an adventure getting down Main Street). But all in all, it was a great day. Spent 4 or 5 hours in MK, got to do rides, got to experience the full WDW Christmas vibe. Can't wait to do it again.
As we left, the line trying to get into the MK parking lot was like nothing I've ever seen. Cars backed up well past the split off to go to Epcot. Insane.
So get there early, be organized, and if you chose to stay later in the day, be prepared to deal with a massive (though festive) crowd.
