July and August are the main hiking months — the high passes are snow-free, the weather is warm, and all facilities are operating. The Mestia-Ushguli trail is walkable from late June, though the Adishi river crossing may be difficult until mid-July. August is the busiest month: book guesthouses on the trail in advance.
June and September offer the same hiking but with fewer people. June can have lingering snow on passes; September is often clearer and the highland meadows are still green. October brings cold nights and the risk of early snow closing the higher routes.
Winter (December-March) is for skiing: the Hatsvali resort is operating, the snow is usually excellent, and Mestia itself takes on a different character. The Queen Tamar Airport flights are weather-dependent all year and particularly unreliable in winter; have a backup plan involving the road.