I often go to Arlon for the marche aux puces, but I'm wondering if anyone knows of any equivalent antiques and flea markets, that sort of things in or around Thionville and/or Metz. Thanks.

no sorry. But I would be interested in hearing where exactly are the ones in Arlon and on what days

They are listed in AngloINFO What's On Guide. See the tab "What's On" above and select "Antiques & Bric a Brac" in the "What" box:

or click here: http://luxembourg.angloinfo.com/whatson/select/6-33-0/shopping/antiques-and-bric-a-brac/in-luxembourg/from-today

Let us know if you know more: http://luxembourg.angloinfo.com/whatson/newevent/

We haven't been to it yet, but I've heard several colleagues talk about the flea market in Metz. Supposed to be pretty big. Here's something on the Metz tourist website: http://tourisme.metz.fr/en/metz/festivite/printemps.php