May and June are excellent: warm and clear, with the Rhodope Mountains still snow-capped in the distance, wildflowers on the surrounding hills, and the city's outdoor life beginning in earnest. Accommodation is easier to find than in peak season.
July and August are hot — Plovdiv sits in the Thracian Plain and summer temperatures regularly exceed 35°C. The Roman theatre outdoor concert season peaks in July and August, which is a reason to be there despite the heat. Stay hydrated and plan activity for mornings and evenings.
September and October are the best months. The heat has broken, the wine harvest is happening in the surrounding hills, and the Kapana quarter is lively without being overwhelmed. The Plovdiv International Fair in September is a major regional trade event.
Winter is cold and grey, with snow possible. The old town and Kapana continue to function; the outdoor sights are less appealing. For a long weekend city break, winter is perfectly viable — hotel prices drop significantly.