Great Barrington is genuinely good in three seasons — summer, fall, and winter — with spring being the one period that can be muddy and grey enough to disappoint.
Summer (June–August) is the main season. The Berkshires Arts Festival runs in early July; Tanglewood (Boston Symphony Orchestra's summer home, 20 minutes north in Lenox) fills the calendar with concerts through August. Hiking trails are at their best, and the town is lively. Book accommodation well ahead for summer weekends.
Fall (September–October) is spectacular. The foliage in the southern Berkshires peaks in mid-October, and the combination of autumn colour, comfortable temperatures, and slightly fewer visitors makes this many people's favourite time. The Appalachian Trail is excellent for day hikes.
Winter (November–March) brings skiing: Butternut Mountain is within easy distance, and several larger resorts are in neighbouring Vermont. The town is quieter but not dead — a well-functioning Main Street keeps going year-round.