Open houses are more for the agent finding new buyers than actually selling the house itself.
If the gates are closed, how will citizens be able to attend the Open House?
What if the promotion was successful and you had 20 marvelous attendees. Where would they park?
If the open house is tomorrow...there is very, very little you can do now. Make sure all the residents are notified and you've placed announcements in all of the public spaces, elevators, lobbies, bulletin boards, laundry rooms, recreation building, and fitness center. Be aware, all of these folks have a place to live. Be clear regarding the strategy in play to make this beneficial to selling your property.
Of course, there is a solution to each of these issues. Chat with your agent to make sure provisions are made before the next open house that can and should be broadcast beyond the gates.
The only trace of you that should be present during an open house is the portrait hanging in the hallway to the library. Take care that a good back story to that portrait is avialable otherwise take it down also.
How to advertise?
Depends of if you want to be on the entertainment circuit or want qualified buyers to attend.
If the latter is the goal, let your agent broadcast through the professional network that represents 99% of qualified buyers. Make the 'invite' the vessel for anticipation.
Best of success to you.