Hi, Rachel!
Great question!
When I uber-plan our trips to Walt Disney World (what other way is there to do them, right?) I don't base my Advanced Dining Reservations on Extra Magic Hours, but on how our ADRs coordinate with each other. For example, our tradition is to begin our trip with a visit to Magic Kingdom, so I'll book an ADR for an early dinner either in MK or at a monorail-accessible resort. This will ensure that we won't feel that we're being pulled away from park touring too early (i.e. lunch) and will have time for nighttime activities at MK after a great dinner.
I'll then decide if we'd like to try a different park on Day 2 of our trip, and will book an ADR in that park for the next day. We try not to book an early morning breakfast the day after a late night, too; our Animal Kingdom days are always shorter, so I'd book a resort ADR for that early that night and / or an early a.m. breakfast on the next day.
If you'd like to try to align your reservations with
Extra Magic Hours which have not yet been released, take a look at some unofficial sources of information on the Internet. For example, I subscribe to touringplans.com, an unofficial crowd-predicting site which includes
predicted Extra Magic Hours in its plans. For other options, I Googled "extra magic hours December 2013" and came upon a PDF entitled "Walt Disney World Operating Hours*December 2013" (also
unofficial.)
Keep in mind that all EMHs, even once released, are subject to change, so it's best to make your plans with those hours as one of only several reasons for your decisions. If they change, then you'll still be happy with your ADRs.
Happy planning, Rachel! Be sure to send more questions as you think of them, too; we love to help!