Rick Steves says Sunday is best to walk around Le Marias.
Paris Walks offers walking tours only on Tuesdays and Saturdays.
If I take the Saturday tour, what will I be missing?
Some stores and restaurants will be closed because it's the Jewish Sabbath. I'd still do the Saturday walking tour.
Sunday in the Marais is a bit of a zoo because everyone goes there. It gets packed wall-to-wall strollers. Not much fun. A walking tour would be much more enjoyable on a Tuesday or Saturday.
Sunday is a good day to go if you want to visit the Bastille Market
https://parisjetaime.com/eng/shopping/marche-bastille-p2267
If you are taking the Paris Walks tour then Saturday is fine. As mentioned, some of the shops will be closed on Saturday.
Please note it is Marais, not Marias. Thanks!
Saturday is fine unless you have l'As du Falafel in your targets. The only closures will be the Jewish shops left on the rue des Rosiers.
I would definitely take the tour. The Marais is now filled with trendy shops and eating places, the streets and sidewalks can be jammed. If you want to get any idea of what it was like before it gentrified, you should take the tour.
If you see a line for a falafel place, jump in. You will not be disappointed.