Hi, Julia!
I cannot wait each year for the Christmas festivities to begin! And if you are looking for THE best holiday lights and decorations, nobody does it better than Mickey Mouse himself.
As soon as the Halloween decorations come down on November 1, the Christmas decorations start to go up. While the transformation into Christmas mode will not be complete until Thanksgiving, you will see the parks and resorts getting more and more festive by the day! Last year, we arrived at Port Orleans on November 17. There were no decorations at our resort. We woke up the next morning to find our lobby and grounds completely decked out with beautiful decorations everywhere!
The first place the "elves" decorate is the Magic Kingdom as
Mickey's Very Merry Christmas Party begins in early November so the park needs to be festive. From there, just like magic, all of WDW turns into a Christmas Wonderland!
For the full effect, including the Candlelight Processional at Epcot, you will want to wait until after Thanksgiving. But starting in early November, little bits of Christmas cheer will start to appear! Have a very Merry Christmas!