Nepal is mostly a subtropical country, with only the mountain areas experiencing the usual season. The main travel seasons are from late September right through to December, and then again from April to early June.
The monsoon is from June to August. In the Kathmandu valley is usually not as intensive as in other areas of the country. During monsoon the valleys are green, the rice fields are beautiful, and there are less tourists here, a perfect time for a quiet time or a retreat at Kopan.
Find more information with details on temperatures here http://www.lonelyplanet.com/nepal/weather