Peak Season to visit Nepal is from late September to late November, when the weather is dry and clear, and it is neither too cold or hot.
Shoulder Season: March to April, spring season, sees warm weather and rhododendrons flowering in the hills. It is a good time to see Nepal and its wildlife, and also trek on its trails.
Off-season: Monsoon season, from June to September, brings with it heavy clouds that limit mountain views and landslides that block trails and paths. If your flight isn’t cancelled and you manage to make it inside the country, then you are rewarded with massive discounts on hotels and accommodation.