When you return to the starting chamber, use the door marked in the picture and go through the corridor and into the neighbouring room.
Afterwards, go to the next one, where you must use another doorway in order to reach the room that is your target.
When you get there, make sure that it is not later than 8.30 pm. As during the later hours there are no chances to save the victims, and the last two stages will have to be repeated. Otherwise, interact with the switch, that has been marked in the picture above. Around 8.45 pm a cutscene will begin, showing that the guests have been rescued.