If one person/group goes each week starting from now, and the final has two people/groups in it, the final will be on Dec 11th. The single is being released on Dec 20th, meaning it WON'T be Christmas number one, since the next chart after that won't come out until Boxing Day, but it will be one of very few new singles released that week. They couldn't possibly have the final on Dec 18th and still get the record out on the 20th, so it does look like Dec 11th is the day of the final, unless it's even earlier and they speed up the process by having a double elimination, or more than two in the final.
Apparently, they were originally going to release a week earlier, hence going for the Christmas number one, but now they aren't. Whether or not they've bottled it, or there's another reason, who knows. I'm sure I heard when all this started that the first live show was going to be on Oct 16th, so now that it's Oct 23rd instead, it shows that they've moved the whole thing back a week.