Note: logging in is not possible for passengers who added special assistance to their booking (e.g. passengers who are fully wheelchair-dependent, hearing impaired or visually impaired, unaccompanied minors etc.). In that case TUI reserves the seats and you are requested to go to the check-in desk at the airport.
If you want to make a seat reservation until 28/10 for flights departing on 29/10 or later, you should take into account the time difference (winter time).
E.g. For a flight from Brussels to Alicante departing on 20/11 at 12 noon, it will be possible to reserve a seat at the earliest 30 days before the flight at 1:00 pm.