I think you are confusing two totally seperate events. You would have to have a seperate ticket to attend the Mickey's Very merry Christmas Party which is offered on select nights from 7pm-midnight. The park closes to day guests and then the party begins, in order to stay for the party (or enter for the party) you will have to have a Party ticket which is seperate from the regular day passes.
The filming of the Christmas Parade is usually done during normal park operating hours the first weekend in December. For that Main Street will be conjested with onlookers and the "filming crowd", but the park is open as normal. They do not close anything down for the filming, other than portions of the sidewalk on Main Street. They do have a sign up for guests wanting to be a part of the crowd, it is not just anyone that happens by, but the park will be open. They also do not run the parade fully through during the filming, it is a slow tideous process, one float at a time, which can be shot over and over again, backing up and doing it again and again. In past as a thank you to the day guests for putting up with the chaos they have run the Christmas Parade in place of the normal day parade, but that does not always happen.