Generally held in the mornings, though in July and August non-food related stalls tend to stay all day.
Monday : Beynac, Les Eyzies 
Tuesday : Cénac
Wednesday : Sarlat, Montignac, Martel, Hautefort, Cadouin,
Thursday : Domme
Friday : La Roque Gageac, Souillac 
Saturday : Sarlat, Montignac, Belvès, Martel
Sunday :  Carsac,  Daglan (summer truffle market), Saint-Cyprien

In high season, best to get there early to avoid the crowds and find somewhere to park.  This is especially true of Sarlat on a Saturday and Saint-Cyprien on a Sunday.