We are considering Paris and Alsace the week of Thanksgiving, but I'm concerned it will be too early for decorations and Christmas markets. Would it be better to wait a week. Thanksgiving is late this year.
Not sure about decorations, but a number of Christmas Markets will be opening on November 24th this year, Strasbourg does. Others may wait until the following Sunday.
In Paris, there are a number of markets, some opening mid-November, some not until Mid-December. This site includes Markets and other December events:
https://www.parisdiscoveryguide.com/paris-events-december.html
If the Strasbourg markets are already open on your dates, I say go for it. The earlier in their season you go, the less crowded they are.
Whether the Paris "markets' are open then is immaterial, as they suck !! You would go to Paris just to be in Paris, but not for the Christmas markets specifically.
I’ll also add that, if you do go, try to go mid-week if possible. The weekends, at least in Colmar, were insane on the weekend we were there this past Christmas. I promised myself I’d never go on a weekend again.
Agree with HappyToBeHere on that point !