Spiti Valley is essentially cut off for much of the year. The Manali route via Kunzum Pass is open roughly from mid-June to mid-October, with precise dates depending on snow clearance and monsoon conditions. The Kinnaur route via Shimla and Reckong Peo is navigable year-round in theory, but road closures due to landslides and rock falls are common throughout the year and nearly unavoidable in monsoon season (July-August).
The best months are mid-June through September. July and August bring warmer temperatures (15-20°C in Kaza during the day) but also the risk of road closures from rockfall, since the valley sits in a rain shadow and receives very little precipitation from the monsoon. September is generally considered the best month: stable weather, clear skies, and the landscape has its maximum colour from the brief summer growth.
Winters are brutal — temperatures below -30°C at higher elevations are routine, roads are buried under snow, and most guesthouses close. A small number of travellers specifically visit in January-March for snow leopard tracking, which is the best time to spot them but requires serious preparation and warm gear.