Nepal has two main trekking seasons.
From February to May and late August to December. Though Nepal remains an all year round destination for biking, sight seeing, meditation, paragliding and many other activities.
Feb and March are great times to trek as the days are bright and sunny and the rain fall is minimum. At night time it can get very cold and some higher places still experience snow. Its a great time to see the villages preparing for the spring cropping season and to see the mountain areas in snow. A t this time the peaks have a lot of snow and make for some stunning scenery.
From March is coming the spring time bringing milder days and nights, though it can still be cold in the mountains. During spring Nepal is at its pretties with wild flowers and rhododendrons in full bloom and the rivers flow crystal clear from the snow melt. Its also an industrious time for the villages and a great chance to see the traditional way 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. Its a great time for trekking as by now all the trails are clear of snow. Occasional rain fall and persistent clouds make the weather a bit humid.
March, April and May are also the high season for peak climbing.
June, July to mid August, is the monsoon season in which the Nepal experiences its most heavy rain fall. Trekking at this time is possible in some low altitude areas, if you dont mind getting wet and in dry areas like upper mustang and the Dolpa. Road travel can be quite difficult in remote areas during this time due to landslides and road closers esp. if you plan on coming into Nepal from Sikkim of Ladhak.