This "coffres ouvertes" roughly translates as a trunk/boot sale. It is like a communal garage sale/flea market. Individuals as well as professionals set up their stands from March through November on the 2nd and 4th Saturdays per month.

If you would like to sign up for a stand, then you must be one of the first to send in your reservation as they take first come = first served up to 100 stands. Sign-ups start 15 days before each date.

http://www.entre-particuliers.info