Pre-monsoon season of March April May and pre winter months of September October November are the best time for trekking in Nepal. From mid-September rain starts decreasing making a clear clean sky for the amazing mountain views. As you approach November it starts getting colder and shorter days. High in the Himalayas, temperature can drop sharply at night from 0 to minus 10 or 15 in November.
December January February are the winter months in Nepal. Trekking in winter is possible, the only problem is cold, with cloudy to clear days and very cold nights as sometimes the snow covers as low as 2500m.
March April May has hot days with 25 to 30 degrees Celsius at lower altitudes and the temperature decreases the higher you go. When you go higher than Namche (3,450 meters), the day-time temperatures can vary from 10 to 20 degree Celsius depending the weather is sunny or windy. In the mountains, there will be more snow in these months due to the accumulation of snow from winter. This is the time when national flower of Nepal, Rhododendron is at its bloom making the views very picturesque on the way. This is the time when climbers to Everest do their ascents.
June July August is rainy season in Nepal which usually lasts from mid-June to mid-September. Except some remote trekking routes such as Upper Mustang Trek, Nar Phu Valley Trek, Tsum Valley Trek and few other, it is not possible to trek during rainy season in the popular trekking routes.