The classic season is June to early October. June is good for moose-watching (cows with calves come down to the logging roads), July and August give reliable lake weather and the warmest swimming, and late September into early October brings the fall colour — the hardwoods turn brilliant red and yellow against the dark spruce.
Winter is the other distinct season: snowmobiling is huge in this corner of Maine, with a major trail network radiating from Greenville. Cross-country skiing, ice fishing, and ice climbing all run from late December through March. Days are short and very cold (regularly minus 20°F at night).
Black fly season runs from mid-May to mid-June; bring DEET or stay south. The lake is mostly frozen from December to April — no boat rides.