Spring and Autumn are taken as best season for trekking in Nepal. But visiting Nepal is by your will and what time your vacation creates time to visit here. Trekking in lower region can be done any time of the year. But if you wish to go to higher altitude then it is suitable during September to November and from March till May.
The remaining six month are also good but, you may face rainfalls during June-August and can be quite cold with snow falls during December-February.