The weather is probably the best guide for deciding when to plan your trip to Nepal. From mid-September to November are considered the best times of the year. The monsoon will have just ended, and clear skies with optimal temperature will prevail. The main festivals of Dashain and Tihar (Hindu equivalent of Christmas in terms of festivity) fall during these months. This is also the busiest tourist season, and the main tourist centers and trekking trails tend to be crowded with travelers.
The tourist flow decreases a little, but not significantly, between the winter months of December and mid-February. During this time the morning and night are cold and the days are warm when sunny. There might be occasional snowfall at higher elevations.
From mid-February to May is another perfect time to visit Nepal. This time is spring. The temperature is mildly warm in low lands while moderate in higher altitudes with plenty of opportunities to have a tryst with the mountain views. It is also the time for flowers to blossom and the national flower of Nepal – rhododendron sweeps the ascending altitudes with its magnanimous color and beauty.
From June to mid-September, it’s the monsoon, during which time it rains almost every day in most of the part except in the rain shadow zone of Mustang, Dolpo, and Humla. Most of the Himalayas are hidden behind the clouds.
In short, plan to visit Nepal between mid-September and November and from mid-February-May are the best times.