Paris - European Heritage Days Sept 20-22

You need to check to see what hours of the days of that weekend are free to
everyone. Once you know that, I would say to stay away from most major
museums during the free hours. It will most likely be a madhouse and you'll
indeed stand in line for quite a while. I do not believe there are reservations
taken for the special free weekend hours.

Just thinking out loud, but museums that are free anyways might be worth a
shot. But apart from museums, the big draw is that a lot of government
buildings which are not normally open to the public will be during that weekend.

I believe there is a website which lays out all the details of what's open and when.

Saint Chapelle has availability on 9/20, so I would make reservations for that day if that day has time for sightseeing. There are tickets available for both the Louvre and Musee d'Orsay on all 3 days on their official websites, so I'm not sure what you are seeing vs what I am seeing.

We had tickets for Saint Chapelle in April and it was still chaotic, so add in free admittance or no reservation...no thanks.
We also advance tickets for d'Orsay and again very busy, but it was during the Impressionist special exhibit. The Louvre on a free admittance day sounds overcrowded.

janettomko,
Could you convince your friends that Paris would be easier to navigate during the week? Since they expect you to make all the plans (I have done that for family/friends more times than I wanted), perhaps they should not be adamant about Paris on a weekend. As the previous answers say, things could be packed when free and they could leave Paris with less happy memories.

(I don't even go toplaces where I live in Southern California on free days, except maybe a concert in a park.)

Thank you for your replies.
We have booked flights (long ago), flying into CDG Friday morning for a few days in Paris before joining a bike tour in Bordeaux. We have train travel all set between Paris and Bordeaux, hotel secured, etc.
These friends are not big museum people - maybe I will book some walking tours in various neighborhoods!!

There is an official site, but as you might guess for anything coming from the
government, it appears hopelessly complicated to navigate.

Here is a site that has descriptions of everything that is going on, by
arrondissement:

Heritage Days

As one small example, Paris City Hall, which is usually not open for individual visits
any more, is open on Saturday and Sunday. Saint Chapelle is free only on Saturday and
Sunday.

Note that the title as OP has typed it is "European Heritage Days". This is accurate
in that every country is going to have stuff going on that weekend.

janettomko, just tossing out a couple of suggestions since you mention perhaps they are not museum people, but maybe??...Musee de 'lOrangerie to see Monet's water lilies murals (small manageable museum with some other famouse artists as well, but can make it a quick stop and it's in the Tuilleries). Requires reservations, otherwise admission not guaranteed. And we thoroughly enjoyed the Rodin museum this past spring. We arrived late and so we spent all of our time outside in the gardens surrounded by beauty, sculptures and a view of the top of the Eiffel Tower. So a little taste of the arts without being overwhelmed. Never made it inside and didn't regret it at all; thought it was perfect. (You pay for admission even if you just go into the gardens. No advance tickets were necessary.

Will everyone feel up to (based on jet lag) the Seine river boat ride to time it with the nightly hourly twinkling of the Eiffel Tower lights? I think they start at 9pm? It never disappoints. Otherwise enjoy a daytime cruise.

It won’t change anything at the places where you can buy tickets in advance with a reserved entry time. Museums won’t be letting people in for free at the same time others have paid tickets. So, if a place you want to go will give you a time-slot reserved ticket, it will be like any other day.

Often, what is offered is behind the scenes tours for free, or normally closed offices in beautiful places are opened to the public on a first come, first served basis. You’ll see the lines outside. As stated above, Paris city hall is an example, the National Archives in the Marais is another.

Other sites not generally open to the public will admit people by reservation.
So for the most part, you won’t be affected unless you have a burning desire to visit the Elysée Palace to see President Macron’s office, or the National Assembly or the Senate.