Hi Deborah,

Visiting Walt Disney World when the parks and resorts are decorated for Christmas is absolutely fabulous!  It's my favorite time of the year! 

Historically, Walt Disney World starts to put up Christmas decorations a few days into November.  In fact in the Magic Kingdom, it seems to me that one night there is a Mickey's Not So Scary Halloween Party going on with guests trick-or-treating and the very next day it's Christmas!  Mickey's Very Merry Christmas Party in the Magic Kingdom typically starts around mid- November with the last party in mid-December.  In 2009, the first party was on November 10th and the last was on December 18th.  Typically Disney doesn't release the dates for the parties for the upcoming Christmas season until late spring or early summer.  You can check here.

Also be sure to see the Osborne Family Spectacle of Dancing Lights at Disney's Hollywood Studios.  It is an amazing display!  It's first night is typically before Thanksgiving.  In 2009, the first night you could see them was November 10th.

Enjoy planning your magical vacation!