Nepal has four seasons. It can be divided into four main seasons, autumn, winter, spring and summer (Monsoon). Mostly travelers prefer to travel Nepal during late September to early December when weather is perfect so that they can get clear view of the mountains. Thousands of people buckle-up themselves to hit the trails to mountains like Everest, Annapurna regions, Langtang at this time.
This is also the season of festivities as Nepal celebrates the biggest Hindu festivals Dashain followed by Tihar. You might need to work hard to book the resorts/hotels as this is the most peak time in Nepal.
If you prefer to travel in lower end region then you can also travel from the end of February to mid-April. It is best time to see wildlife in Chitwan and Bardia national parks.