We're also headed out on a December/January Disney trip. Let's hope the weather is good!
This is a great time to make up a packing list, so you'll have plenty of time to shop for anything you might want to get before you go. Since we leave on the 27th of December, and only have the 26th to pack, I have a very detailed list completed well before that date, with all the necessary laundry done and any last minute things (sunscreen, moleskin, etc.) purchased.
This is also a good time to make out a more detailed plan (unless you plan to wing it), outlining which parks you plan to see on which days, ADRs, and evening events. I actually only make my ADRs after I figure out which park we'll go to, to avoid too much travelling. I make my plan with a table in WORD with 6 sections for each day, as shown below. For each section I add as much info as I need. Sometimes it's as simple as "5:45 p.m. - Mama Melrose - ADR# 1234 5678" or "hotel food court". For other sections I outline which rides I hope to see, and reminders (e.g. that we have to see Splash Mountain on our first MK day because it will be closed on our second MK day).
It's a huge timesaver if you know that you'll head directly to Splash Mountain, for example, rather than waiting to pull out a park map after you arrive and put it to a family vote.
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Sample Plan
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Friday, December XX, 2006 - MGM Day
Breakfast:
Morning:
Lunch:
Afternoon:
Dinner:
Evening:
Have a wonderful first trip!