Trebinje has hot, dry summers and mild winters, with the Mediterranean influence coming up from Dubrovnik. May to October is the long warm season; July and August can be very hot (35°C+), with the locals retreating to cafes by day. September and early October are perhaps the ideal months — warm days, cool evenings, wine harvest at the surrounding monasteries.
November to March is the rainy season; the bura wind from the north can push the temperature down sharply, and the town is much quieter. The Trebinje Summer cultural festival in July fills the squares with music, theatre and poetry readings (in honour of Jovan Dučić, the local poet); it is the best time to visit if you want to catch a sense of the town at full social tilt.