October and November are by some distance the best months to visit Patan. The monsoon has cleared, the skies are dry and bright, the temperatures are mild, and Dashain and Tihar — Nepal's two biggest festivals — fall in this stretch. February to April is the next best window, with rhododendron blooms in the surrounding hills.
The monsoon (June to September) brings hard, daily rain. The valley turns lush, the crowds thin out, and prices drop, but heavy rain can make sightseeing miserable. Winter nights are cold and most buildings are unheated. Mornings can be foggy until late.