Summer (June to August) is peak season — Plimoth Patuxent is fully staffed, Mayflower II is in service, and the beaches are swimmable. The waterfront is busy and parking is tight on weekends, but the town handles the crowds reasonably well.
Autumn is arguably the best time to visit. The crowds thin after Labor Day, the weather stays mild well into October, and the cranberry harvest (late September to October) turns the bogs south of town a vivid red. Foliage color typically peaks in mid-October.
Thanksgiving week is Plymouth's busiest period of the year, for obvious reasons. Plimoth Patuxent runs special programming, but expect larger crowds and higher prices everywhere in town.
Winter closures apply at Plimoth Patuxent — the outdoor museum typically shuts from late November through March. Plymouth Rock and Burial Hill are always accessible, but there is less to do. The waterfront restaurants thin out considerably.