We haven't heard some a few of the people we sent invitations too. Our RSVP due date is July 23rd. When after July 23rd is it okay to contact the people as to if they are coming to the wedding or not.
I understand not everyone can make it, but want a good idea of who and how many will be coming. We are also having assigned seating, so this could become a problem if people show up who didn't RSVP.