Auli has two distinct seasons and a middle period that's best avoided.
Winter (December to March) is ski season. The slopes are operational when snowfall has been adequate, which is most reliably the case in January and February. Temperatures at 2,500 metres are cold — well below zero at night — and the gondola is the only practical way up when conditions are snowy. Accommodation books out during peak skiing weekends in January and February.
Spring and summer (April to June) bring warming temperatures and the meadow wildflower season. Gorson Bugyal and the approaches to Kuari Pass are at their best in May and June, before the monsoon arrives. The views are typically clearest at this time of year.
Monsoon (July to August) is when most visitors should avoid the area. Heavy rainfall makes trails slippery and visibility poor. The roads can be disrupted by landslides on the approach through Chamoli District.
Autumn (September to November) is excellent — clear skies, good visibility for mountain views, comfortable temperatures, and huckleberry season in the forests. This is the second trekking season and arguably more reliable than spring.