On your listing page you'll find a field marked Open House Info. You can enter the time and date of your open house there and it will display on your tour. Once the date has passed, it will automatically be removed. You may also consider typing it into your script for the TTS voice to read. That way when someone is listening to that tour, they hear it as well. That however will not auto update so you will want to remember to remove it from your script once your open house has passed!