Nepal has two main trekking seasons. From February to May and late August to December. Nepal is an all year round destination for biking, sight seeing, meditation, paragliding and many other activities.
February and March are great times to trek as the days are bright and sunny and the rain fall is minimal. At night time it can get very cold and some higher places still experience snow. It is a great time to see villages preparing for the spring cropping season and to see the mountain areas in snow. At this time the peaks have a lot of snow and make for some stunning scenery.
March heralds the coming of spring time, bringing milder days and nights, though it can still be cold in the mountains. During spring, Nepal is at its prettiest with wild flowers and rhododendrons in full bloom and rivers flowing crystal clear from the snow melt. Its also an industrious time for local villagers and a great chance to see traditional ways of farming.
In April and May the weather begins to get hotter and the days can be quite hazy, though on the mountain side the evenings remain cool. It is a great time for trekking as by now all the trails are clear of snow. Occasional rain fall and persistent clouds can make the weather mildly humid.
March, April and May is the high season for peak climbing, including Everest.
June, July to mid August is the monsoon season in which Nepal experiences its most heavy rain fall. Trekking at this time is possible in dry areas like Upper Mustang and the Dolpa, and in some low altitude areas, if you don’t mind getting wet. Road travel can be difficult in remote areas because of landslides and road closures especially if you plan entering Nepal from Sikkim or Ladhak.
From late August the monsoon begins to dwindle and the sun comes out again. The days are warm and clear. September onwards offers the best and clearest time for viewing the mountains. This is an ideal time for trekking as the weather is warm, and during the days T-shirts and shorts are adequate. In the lower areas, expect the weather to be quite hot.
In October the weather begins to cool, especially in the evening. Anywhere from the end of October, snow can be expected. Even so, it is the best time for trekking as the air is clean and clear and its generally sunny. Clear views are mostly guaranteed.
In November and December the trekking is still very good though in the high passes and altitude areas, you should be prepared for cold and snow.
December and January are the coldest parts of the year. The mountains down to about 2,500 meters get heavy snow and the high passes are mostly closed for trekking, as are routes like Annapurna base camp and Everest base camp. It is possible in these times to do lower altitude trekking around Kathmandu and Pohkara, solukhumbu district and Ghorka. In fact it is a good time of year to do these kinds of treks as the days are dry and sunny without being hot, though the evening are quite cold. Normally the views are good.