The answer is almost always winter.
Winter (November–March) is Yabuli's reason for existing. The snow is reliable from November, with the best conditions typically in January and February when the snowpack is deepest and temperatures are consistently cold. Chinese New Year (usually late January or early February) and the Golden Week holidays in October and May bring peak crowds; mid-week in January and February is notably quieter. Temperatures at altitude reach -20°C or lower — dress accordingly.
Spring and autumn are the transitional seasons. The snow melts from the lower runs in March and is gone by April. September and October see the first frosts and the birch forest turns gold, making the mountain attractive for day hikes even if the ski runs are closed.
Summer (June–August) is warm and green. The resort operates in reduced mode with hiking trails and limited lift access for mountain biking. If you want mountain scenery without the cold, this works. But most visitors come specifically for the skiing, and the resort's identity is so winter-focused that summer feels slightly hollow.