There are a lot of annual events that happen in Paris. Most of them are pretty famous, but I suppose if you don’t pay a lot of attention to Paris (outside of the main tourist’s seasons) you’ll probably miss most of them.
One such famous event is Nuit Blanche or Museum Night in English. Every October the museums of Paris open their doors for free until midnight or later. The lines are long, but it can be worth it. One year Beaux Arts was open to the public for instance.
This year, unfortunately, it was not good. The terrible website didn’t work, and the main event was a run that had a people limit and limited registration dates. But in previous years there have been parties, and food trucks, and events happening everywhere. I would definitely recommend Paris in October, and I would definitely recommend it to take place around this particular Saturday. Even if it’s crap like it was this year, you’re still guaranteed to have an experience.