Lormes sits at around 400 metres elevation on the western flank of the Morvan, which means cooler summers and harder winters than the Burgundy wine towns to the east. Late spring through early autumn is the most comfortable window.
May and June are excellent: the forest is deep green, the rivers are running well for kayaking, and the countryside is not yet crowded with summer visitors. July and August bring the warmest swimming weather for Lac de Chaumeçon and the Étang du Goulot, and the town hosts its Festival de la Chanson Française on the last weekend of July — a well-regarded event that draws nationally known performers to the town square, entirely out of proportion to Lormes' size.
Autumn (September and October) is arguably the finest season in the Morvan: the oaks and beeches turn in the gorges, the light is low and golden, and the crowds thin out sharply. Wild mushrooms are everywhere, and the weekly market fills with local produce. Winter is quiet to the point of dormancy; several businesses close and the campsite shuts. Unless you want the Morvan entirely to yourself, there are better months.
The weekly market takes place in Lormes on Tuesday mornings. If you time your visit to coincide with one of the rotating Comice Agricole fairs — an old Burgundian agricultural tradition involving costumed parades and flower-decorated floats — you'll see a side of rural French life that has changed very little in a century.