Hanover is at its most alive during the academic year, from September through May. The Hopkins Center programming, the Hood Museum, and the general energy of the campus are all in full swing. Summer is quieter — not dead, but slower.
Fall (September through October) is the best season. The Upper Valley's foliage peaks in early-to-mid October, and the combination of brilliant color, crisp air, and a full campus is hard to beat. Leaf-peepers arrive in number, so accommodation fills quickly.
Winter is cold and snowy, but Hanover handles it with Nordic cheerfulness. Dartmouth's Winter Carnival, held in February since 1911, is one of the oldest collegiate winter festivals in the country — ice sculptures on the green, ski races, bonfire. The skiing at nearby Dartmouth Skiway is modest but reliable.
Spring brings thru-hikers. The Appalachian Trail's northbound "bubble" passes through Hanover in late May and early June, and you will find hikers in various states of trail-worn glee restocking at the co-op and sleeping at the church hostel.
Summer is warm and green, and the Connecticut River is swimmable upstream at several points. The town is quieter without students, but the surrounding Upper Valley — farms, covered bridges, the river — is at its most pleasant.