Bamiyan sits at about 2,500 metres in the central Hindu Kush, and the climate is determined by altitude.
Summer (May to September) is the only practical season for most visitors. Days are warm and clear, the valley is green and the apricot trees are in fruit, the road to Band-e Amir is open, and the air is dry. June through August is the peak.
Winter (November to March) is severe — heavy snow closes the high passes between Bamiyan and Kabul, often for weeks at a time, and the cold drops well below -20°C. Don't try to drive in.
Shoulder months (April, October) are unpredictable — sometimes warm and clear, sometimes snowy.
The annual Buddha Day events around 22 March (Nowruz, the Afghan New Year) draw local crowds to the cliff. Security conditions across Afghanistan can change rapidly; check the situation close to your travel date.