If you have a choice, go for October to December, when there’s not much rain and visibility is good. It gets gradually cooler but never gets very cold at biking elevations – in fact, even in December and January the days can be sunny and even warm anywhere up to about 3000m, though snow may occasionally be encountered as low as 2000m.
December and January are also the most comfortable months for cycling in Pokhara and the Terai. The shortening days are a factor, though: by December you’ll need to be off the roads or trails by 4.30pm or so.
From January to March the days lengthen and grow warmer. This too is a good time for biking. In April, May and the first part of June, the weather keeps getting hotter, the road conditions dustier, the air hazier – and afternoon showers become more common. On the plus side, you can take advantage of long daylight hours. The monsoon (mid-June to late Sept) is hot and damp; the mountains are usually hidden by clouds, and the trails are wet or muddy. This is prime riding time in Tibet and Mustang, however, both of which are shielded from the rains by the Himalayas.
Another seasonal consideration is the race calendar. Events run by different mountain-bike companies are held throughout the year, but some have a serious international profile. Yak Attack, organized by Dawn Till Dusk, usually takes place in early March, and takes serious competitors from Kathmandu west via Nuwakot and Gorkha (off the main roads) towards an incredible crossing of the Thorung La pass, in the Annapurna range. The Trans Nepal race, in December, follows a 5-day route on 4WD trails from Kathmandu to Pokhara.