My family has gone the past 8/9 years.
Honestly? It depends.
This past year, Christmas fell on a Monday, so a lot of people arrived on 12/23 and stayed through the holiday. When Christmas falls mid-week, you'll see an uptick start around 12/24-12/25. 12/25 is obviously crowded, but it doesn't get especially crowded until the days after Xmas, when kids are officially on break.
If I had to bet, I'd say it'll get crowded as early as 12/21 in 2018, with Christmas falling on that Tuesday. On a rate from 1-10, here's how I see the crowds breaking down:
12/21: 7 - most guests you encounter here are either checking out after visiting Disney for a pre-holiday-week trip. Maybe some checking in for Christmas.
12/22: 7 - A big check-in day. If Free Dining is offered, most will try and check in on 12/22. Crowds will get very crowded in afternoon as people check in.
12/23: 7.5 - A slight uptick compared to 12/22. Again, it'll depend on when Free Dining ends.
12/24: 8 - Most people will be checked in for the holidays.
12/25: 10 - All the guests there for Christmas + all the locals celebrating the holiday
12/26: 9.5 - The Christmas Hangover
12/27: 8 - It'll be crowded. Most people visit for Christmas or New Year's Eve. This is when you'll see a slight lull in the crowds. With 12/27 falling on a Thursday, most will choose to fly on this day, as Thursdays are usually less expensive.
12/28: 8 - Crowded. Again, some will be checking in
12/29: 9 - It's a weekend + the holiday season. By now, most NYE guests are checked in
12/30: 9.5 - Heavily crowded. MK puts on its NYE show. Most runners are checking in for the marathon.
12/31: 10: The most crowded day at Disney
1/1: 9.5: Heavily crowded. More runners are checking in
