If you have any choice at all, take the first two weeks
Everything that makes a December trip worth taking — the decorations, the castle lighting, the seasonal parades and shows, the Festival of the Holidays, the party nights, the overlays at both resorts — is fully installed and running by the start of the month. None of it improves in the final two weeks. The only thing that changes is the crowd and the price.
That makes early December the clearest value decision on the seasonal calendar, and it is available to anyone whose school constraints are soft or absent. Moving a trip from the week after Christmas to the first week of December typically halves the room rate, drops the ticket tier, and removes the phased-closure risk entirely.
The second week is nearly as good and is often better for weather, since the coldest fronts tend to arrive later. Both are far better than anything after the third weekend.
The holiday week needs a different plan, not a harder one
If your dates are locked to the holiday period, the standard advice to arrive early and use line-skipping is necessary but nowhere near sufficient. The specific risk is phased closure: Magic Kingdom and Disneyland Park can stop admitting guests entirely by mid-morning during the holiday week, including guests holding valid dated tickets for that park. Being on property and being through the gate before opening are the only real protections.
Restructure the trip rather than the day. One park per day with no hopping, a full midday break out of the park, dining reservations booked at the earliest possible window and taken at off-peak hours, and an acceptance that four headliners is a good day rather than a disappointing one.
The other adjustment worth making is expectations about transportation. Bus and tram queues during the holiday week routinely run 30 to 45 minutes at park close, which is a poor way to end a very long day. Leaving before the final fireworks rather than after is worth more time than any other single decision.
The party tickets are the least bad way to see Magic Kingdom at Christmas
Mickey's Very Merry Christmas Party is a hard-ticket event with capped attendance, running for several hours after the regular park closes. During the holiday season it is the only version of Magic Kingdom that is not operating at its ceiling, and the ride waits during a party can be a fraction of what the same park posted that afternoon.
The nights closest to Christmas are the most expensive of the run and sell out earliest, and the party season typically ends shortly before Christmas rather than running through it — so the last few nights are the scarcest resource in the whole December calendar.
Disney Jollywood Nights at Hollywood Studios runs a shorter season on the same model and is the newer of the two, which historically means slightly easier availability. It is a different evening in tone — later, more adult, more show-driven — and it suits a group without small children better than Very Merry does.
New Year's Eve is a capacity event rather than a party
Both resorts run their New Year's fireworks packages on the holiday, and both close the affected parks to entry early in the day. Standing in Magic Kingdom's hub or on Disneyland's Main Street on New Year's Eve is an endurance exercise, not a celebration, and it caps a week that was already the hardest of the year.
The workaround that most people do not know about: Magic Kingdom has historically run its New Year's Eve fireworks on the night before as well as on the night itself. Seeing the same show a night early, at a fraction of the crowd and without the entry restrictions, is the single best trade available in the holiday week.
The equivalent move at Disneyland is to take the holiday content on a weekday earlier in the period and to spend the holiday itself at Downtown Disney or at the hotels, where the same evening is survivable.
The decorations come down later than you think, and January is the cheat code
Holidays at the Disneyland Resort runs past New Year's and into the first part of January, which means a January trip still gets the seasonal ride overlays, the castle lighting, and the holiday decor at January prices and January crowd levels. Almost nobody takes this trade and it is the strongest one on the whole calendar.
Walt Disney World keeps a good deal of its holiday decoration up into the first week or so of January too, though the party season and several seasonal shows have ended by then. The decorations survive; the ticketed content does not.
If the reason you want a December trip is that you want to see the parks decorated, and not that you want to be there for the holiday itself, book January and save the difference. It is the same photograph and a completely different trip.