I believe the "official" start time is 7pm. However, the guests that are in MK for the day don't have to leave, and as of last year, if you entered the MK a little prior to the "official" start, you were allowed in on just your
MVMCP ticket.
We entered the park on the MVMCP ticket only at 4:30ish last Dec, with no problem, no line!
The crowds in the park prior to 7pm are moved to one area (it was Tomorrowland) and as you have a chance to see a show (Mickey's Twas the Night Before Christmas) and then get in line for pictures before the 7pm beginning, getting in the park early is a great idea!
After the show and getting our pictures taken (all before 7pm) we headed
up Main Street to the main entrance of the Emporium. The line of people entering the park and heading to Tomorrowland was all the way from Tomorrowland to the Main Entrance of MK! The one entire side of the street was wall to wall people! I was THRILLED not to be in that line!
Edited to say that we did NOT have dinner PS's at any MK restaurant that night.