From the enumeration of the duels we know that André-Louis' first day was a Monday (e.g., Chapter 3.IX says that Chabrillane "was killed on Tuesday last" which was the next day).  And he himself declares it to be "some three weeks" after the death of Lagron [Chapter 3.VII].  The only Monday on which André-Louis could reasonably have made this statement was September 13, three weeks and two days after the duel.