Hi there, Angela!
In November and early December, Mickey's Once Upon a Christmastime Parade is a special part of the hard ticket event
Mickey's Very Merry Christmas Party. However, during the two week lead-up to New Year's Eve, this special parade replaces the "regular" afternoon parade at the Magic Kingdom.
While dates have not yet been released, I can tell you that in 2011 it was performed during regular park hours from December 19 to December 31 (stepping off at 3pm from Dec 19 to Dec 24, and at both 12:30pm
and 3:30pm from Dec 25 to Dec 31 2011). If you are there during Christmas week, you ought to have no problem seeing it at least once!
This parade steps off in Frontierland and winds through the Park, concluding at the Car Barn by the Harmony Barbershop on Main Street.
One excellent viewing location is just in front of the Hall of Presidents in Liberty Square; another, if you'd like to watch the parade come directly towards you, is the concrete island on Main Street where the huge tree stands; you'll want to grab a spot facing Cinderella Castle for that one.
And I would go by "feel" insofar as timing is concerned. When I'm at the Park during a particularly busy time, I'll start checking out my desired location about 90 minutes before the parade is due to start. And sometimes, if the crowd looks pretty thick around that area, I will stake my spot that then. In all likelihood, because this is a very popular tie of the year, you'll need to plan for at least an hour in advance, if not more.
Have a wonderful holiday with Mickey!
See you later, celebrator,
-Deborah B.