Hi Mary!
I think you've hit on two great things to celebrate at Walt Disney World: Birthdays and Holidays! And its even better when you can combine both in a single trip!
The Magic Kingdom usually kicks off the Christmas Holiday season in early November as it beings to have special events called
Mickey's Very Merry Christmas Party. These parties usually being during the first week or two of November and continue until right before Christmas week. Typically, this means that the Magic Kingdom will be sporting most of its holiday decorations about that time.
Other parks and the resorts follow shortly after, with full holiday fare in place by about Thanksgiving. During the month of November, special touches are put in place throughout the world. You'll see everything from giant gingerbread houses, chocolate horses and more. To see a peak of the 2010 decorations, be sure to check out this
post on the Disney Parks blog.
Hope you have fun planning AND celebrating!