May, June, and September are the best months. The weather is warm but not hot, the citadel and museums are open without high-season queues, and the surrounding country is at its best -- spring asparagus, then early-summer goat's cheese, then the September grape harvest in the vineyards along the Indre.
July and August are busy but never as crowded as Amboise or Chenonceau. Tour groups don't usually overnight in Loches, so even high-season evenings stay quiet. Winter is cold, with limited opening hours, but the Saturday market runs all year. The Festival Concerts d'Été brings classical concerts to the Logis Royal in July.