The simple answer likely is that the BBC don't know yet.
The current schedule is:
17:15 BBC News (News)
18:50 To Be Announced (Entertainment)
19:35 The National Lottery: In It To Win It (Game Show)
20:05 Strictly Come Dancing (Reality Show)
20:25 To Be Announced (Entertainment)
21:05 Casualty (Drama)
Now obviously the News doesn't run for that long so the first To Be Announced program is likely SCD and I would guess starting around 18:20. The results are on at 20:25 with the other To Be Announced program as Robin Hood.
Slight change from last year then if this is true - Robin Hood used to be between the two SCD shows.