We present the data below. What you might find surprising is that Christmas is not the busiest day in any of the cases shown. Also, the day after Christmas is the best day to visit for all cases shown after Christmas; it pays to arrive before the rest of the crowds get to town!
Best & Worst Days to Visit Walt Disney World 12/19 to 1/1
Outside of the Magic Kingdom, Christmas Day is typically pretty mild across the Walt Disney World theme parks. After Christmas, crowds really peak around the 28th and 29th. EPCOT and the Magic Kingdom will be very busy on New Year's Eve thanks to special fireworks at midnight to welcome in the new year (parks open until 1 AM). This year, Disney's Hollywood Studios will also be open until midnight on New Year's Eve. Animal Kingdom may be the best choice on this day.
Best & Worst Days to Visit Disneyland Resort 12/19 to 1/1
Wait times across Disneyland Resort are more mild prior to Christmas. After Christmas Day, wait times build until they peak around the 28th. This year, both Disneyland Park and Disney California Adventure will be open past midnight on New Year's Eve. Expect big crowds on that day. If you are up for getting up early on January 1st, crowds should be more mild.
Best & Worst Days to Visit Universal Orlando 12/19 to 1/1
Wait times across Universal Orlando are typically above average prior to Christmas, but really pick up the week after. Christmas Day itself is typically more mild at Islands of Adventure than Universal Studios. Wait times the 27th through 30th have historically been well above average. This year, Universal Studios will be open until 1 AM on New Year's Eve, which will likely lead to the park being busier in the evening than Islands of Adventure.
Best & Worst Days to Visit Universal Studios Hollywood 12/19 to 1/1
Wait times at Universal Studios Hollywood historically build after Christmas and peak on the 28th. This year, the park will be open until 2 AM on New Year's Eve, which will lead to a busier day than last year.
Best & Worst Days to Visit Dollywood 12/19 to 1/1
Dollywood hosts a special event, Smoky Mountain Christmas, through New Year's Day. Unlike most other parks, Dollywood is closed on both Christmas Eve and Christmas Day. From there, crowds will build, peaking around the 28th/29th. The park is not open late on the 31st. Weather can play a significant factor too, since colder weather can impact attraction operation.
Best & Worst Days to Visit Busch Gardens Tampa 12/19 to 1/1
Much of the year, wait times at Busch Gardens Tampa are typically very manageable since the park is more locals oriented than the other Central Florida parks. However, after Christmas, the park historically has its busiest days of the year. If visiting the 26th through 31st, expect well above average wait times.
Best & Worst Days to Visit Disneyland Paris 12/19 to 1/1
Like the US parks, Disneyland Paris sees a surge in wait times after Christmas. One lighter day might be Christmas Eve, which was the lowest of the Christmas season last year. Some of the longest waits can be expected around the 28th/29th.
You can find all of the detailed wait time data for every holiday on the dedicated holiday page or by clicking any of the blue text above.