Hi Angela,
You will be rolling out of the busiest week of the year at the Walt Disney World Resorts. You will still catch some of the holiday magic as it can not all be taken down so soon.
The crowds will be heaviest when you first arrive so go to Epcot to find a little more elbow room. The weather can fluctuate as I have been down there in shorts and I have been fine. But I have also seen the temperature drop to the 40's. I would check the weather forecast prior to you coming down.
As for celebrating birthdays, I love the
Whispering Canyon Cafe to celebrate birthdays. It is loud and fun and the food is yummy. If you want something fancier, head to the
California Grill where the food is great and the view is better.
Make sure you pick up pins at the park you are visiting so that the cast members wish you "Happy Birthday". In addition, you never know what magic you might encounter and let them know you are celebrating when you make your Advance Dining Reservations (ADRs).
Wishing you a magical vacation!
Beth