Your lowest crowds will be before Xmas. They will slowly build during that week, but are really quite manageable. Attendance explodes on Xmas Day and remains extremely high that entire week.
Do as many parks as possible before 12/25. Since you arrive 12/21 you should be okay. If you want to avoid crowds stay away from MK on 12/25 and 12/26. If you want to go to a park on 12/25 I suggest Epcot. Due to its size it seems to handle the large crowds better than the other parks. Plus, many people don't feel it's as "kid friendly" as the other parks so they stay away on Xmas Day. DW and I went to Epcot on 12/25 last year and were able to get a lot done. But, we went early, used FP, made PS, and had no children.
I don't think you HAVE to avoid all parks on Xmas Day. (It's not a bad idea, but with only 5-6 days you may need the extra park time). I would certainly avoid MK, but the others can be handled with some planning. Just understand that with large crowds you will not be able to do everything you want and you will have to wait in line, etc. The parks usually open earlier than normal and you can get so much done in those first 2 hours if you arrive right at opening. Make PS for any sit down meals and use FP. As long as you keep this info in mind 12/25 does not have to be a "wasted" day of no parks.
Good luck and have fun.
