Hi Karen,
I say the sooner the better!!! If you are ready for your 3rd trip then I would definitely go with your December dates. If you have already been to Walt Disney World in June then it would be fun to experience the Happiest Place on Earth over the holidays, because it truly is magical.
The lights, the holiday decorations, special events, and fireworks make December a fun time to experience Walt Disney World.
The weather is cooler at that time of year and just before the holidays it isn't as busy.
With the travel dates you have listed you will be able to attend
Mickey's Very Merry Christmas Party which is such a fun night!!
You really can't go wrong anytime of the year. Choose what is best for your family.
Try not to stress about it! You will have an amazing time!!
Thank you for your question and have a magical vacation!
Carrie