The Black Cat is hosting Muggle Mondays and showing the movies in order. It's free, and there will be Muggle drink specials! Doors open at 8:00, not sure when the movie actually starts so plan to be there just after 8. The Geek Night Out Meetup has also scheduled this, so there should be plenty of Potter fans about.
Directions, parking and Metro information here: http://www.blackcatdc.com/about.html