Happy anniversary!
If you're willing to have an early dinner, you could conceivably eat almost anywhere at WDW and still make it back to the Magic Kingdom in time for the 7:00 p.m. start of Mickey's Very Merry Christmas Party.
I'm not sure how many people are in your group or if you'll have children with you, so it's a bit difficult to make a specific recommendation. However, if I were in your situation, my instinct would be to have a dinner at 5:00 or 5:30 at one of the hotels on the monorail line near the Magic Kingdom. Each of these hotels has wonderful restaurants that are perfect for celebrations, plus they're all less than 10 minutes away from the party.
Here are some great choices:
California Grill - Contemporary
The Wave - Contemporary
Chef Mickey's - Contemporary (if you have kids with you)
Ohana - Polynesian
Citricos - Grand Floridian
Narcoossee's - Grand Floridian
1900 Park Fare - Grand Floridian (if you have kids with you)
You can find out more about these restaurants on the WDW website
HERE.