Not sure what you mean about a trip the 18-21 and opening day (27) - are you asking if your trip dates will be sold out (hoping you already made your reservations) or the 27th - the 27th is still showing availability for single day tickets, including the SoCal ones.
As PP said, often availability opens up the night before. On a launch date I am not sure - perhaps someone who wanted to go on the first day of the Holiday decor could comment.
I may be a weirdo but my two recent weekend visits (Sunday in Oct and Sat in Nov) were actually better than my weekday ones, I think because more than one MKey type was blocked. So I personally would lean toward a weekend in January prior to the 100th, and check the MK block out dates. But that is based on absolutely nothing concrete. LOL