Hi Alexis!
The holiday season is one of the most magical times to visit Walt Disney World, especially Magic Kingdom!
Mickey's Once Upon A Christmastime Parade is normally only performed during
Mickey's Very Merry Christmas Party (which requires a separate ticket), but since you are visiting Magic Kingdom the week of Christmas you're in luck. The parade is shown to all Magic Kingdom guests during the day with no additional ticket purchase. This will typically happen after the last Mickey's Very Merry Christmas Party which takes place this year on December 20th.
This is also one of the busiest weeks to visit Walt Disney World and just be aware that it can fill to capacity. Usually, this will require cast members to deny admission to guests for a period of time due to fire code regulations. I suggest to arrive at the park earlier in the morning and plan to hang out for awhile so you do not run the risk of that happening.
Also, there is the televised parade that is shown every Christmas morning on ABC. I wasn't sure if you might be inquiring about this one, but I figure I'll thrown in this information as well.
Spoiler Alert! The parade that is seen on Christmas morning, is actually taped a few weeks in advance. While I'm not sure of the date for 2013, it is usually done the first weekend of December. While guests are still able to watch this taping from afar, the special roped off areas are for those with special production tickets only.
I hope this helps with your planning and please let me know if you have anymore questions!
Best Wishes!
Mandy