If you're in France Sept 19-20, there are castles/etc open which are normally closed to the public.

Many places which charge admission may also be free that day. I was in Paris for this event in 2013 and can tell you that the locals will be flocking to the various locales as it is a major fete for them. Best to go to the less known sites where the crowds should be smaller.

We have a "free" day in Normandy (based in Bayeaux) on the 19th. Contemplating visiting MSM starting very early in the day and making our way back to Bayeaux via other good stops if we have time. Both Villedieu-les-Poeles and Chateau de Pirou interest us.

Will MSM be a crowded mess, even early?

We're definitely open to the idea of going to the TI when we get to Bayeaux or even Caen (where we pick up rental car) for special sites included with the heritage days. Maybe we should just play it by ear based off their recommendations?

I was lucky enough to be in Paris one year on that special weekend. I stood in line for the National Assembly, the Senate (Luxembourg Palace), and the Hotel de Ville (city hall). It was more than worth the wait at each one.

The place with the absolute biggest lines each year in Paris is the Elysée Palace, the residence of the French president. I admit I've never found the fortitude to go stand in that line! Chani mentions the other biggies.

Heritage Open Days / Tag des Offenen Denkmals / Open Monumentendag etc….

Not only France but most countries in Europe monuments are open in September that are usely closed for public. http://www.europeanheritagedays.com/

Thank you so much for posting this information. I will be in Normandy that weekend, visiting from California, and had no idea about the event.