I expect the BBC to start hyping the hell out of this towards the end of this month or even start of November and that will do the job.
Exactly. The BBC as has already been said don't advertise massively 'till a few weeks before broadcast of a new series or one off episode in this case. We all know when it's airing, so I don't get why there seems to be such unrest over it.
As for when it's being broadcast on the day, my money would be around the 18:30 - 19:00hrs mark, it will also require a bit of juggling of programmes including a very early, or split episode of Strictly, but is that the weekend of Children in Need?