The best months to visit Gorkha are October and November, when the post-monsoon air is clear and the Himalayan views from the Durbar are at their finest. The mountains — Annapurna, Manaslu, Ganesh Himal — are snow-capped but the skies are reliably cloudless, and temperatures at the Durbar's 1,500-metre elevation are pleasantly cool rather than cold.
March and April are the second best period. Rhododendrons bloom across the surrounding hills, temperatures are comfortable, and the views are good before the pre-monsoon haze builds. Spring is also the main trekking season for the Manaslu Circuit, so the trails are more active.
The monsoon (June–September) brings heavy rain, leeches on the hiking paths, and persistent cloud that obscures the mountain views. The Durbar is still accessible, but you will likely see nothing of the Himalayas. December through February is cold at higher elevations — the Larkya La pass on the Manaslu Circuit is typically closed — but the bazaar and lower parts of town remain mild and can be visited year-round.
The Dashain festival (October) and Tihar (October/November) are the most important festivals in Nepal and are celebrated enthusiastically in Gorkha. If your visit coincides with either, expect the bazaar to be particularly lively.