Someone asked me the other day what if any markets are on in this region. There is in fact a market every day, except sunday, around Châteaubriant. Here is a lit to help you:
Monday : Bain de Bretagne
Tuesday : La Guerche de Bretagne
Wednesday : Châteaubriant
Thursday : Pouancé and Nozay (just fresh food)
Friday : Martigne Ferchaud and Nort sur Erdre
Saturday : Châteaubriant (just fresh food), Derval and Nozay
All of the markets are only open in the mornings, normally between 9 am and midday.