I think some people just have problems relating to invites and parties.
I made a themed birthday party this year, sending invitations two months prior just to be sure that everyone would be free. Of course some people could not tell me if they could come or not (I'm talking about close friends. I chose the date to accomodate everyone!) and I got very mad because we are all friends and we rarely get to met each other alltogether so...
Everyone gave you AWESOME tips. Mine is... don't get too mad.
Some people are JUST RUDE.
Or don't care. Nobody likes themed parties in my circle of friends