You will probably notice a significant increase in crowds by the 20th and 21st. I would get in the most important rides by the 21st, then take it a little bit slower the rest of the time. You will always get more accomplished if you make it to the parks about 30 min. prior to opening and push hard till noon or early afternoon. As it gets closer to the 25th, it will get extremely busier (and that is an understatement). You can also schedule certain events or activities the last three days of your trip that are not affected too much by the crowds: Keys to the Kingdom tour, an afternoon at Fort Wilderness, a round of miniature golf, trip to the movies, stake out a site for the night parades and fireworks while someone else gets snacks). In other words, try to plan activities that excludes the massive crowds by Dec. 24-27. Just sightseeing is fun at this time of year because of the decorations and Xmass entertainment throughout WDW. Have fun!