Sandusky is essentially a summer destination — Cedar Point opens in early May and closes in early November, and the Lake Erie Islands run a similar season.
Mid-June to mid-August is the peak, with all attractions open, ferries running their fullest schedule, lake water warm enough for swimming, and the longest park hours. Expect long queues at Cedar Point on weekends.
Late August to mid-September is the sweet spot — the kids are back in school, lines shrink dramatically, the weather is still warm, and the park's Halloweekends programme begins in mid-September with serious haunt productions.
October is the last good window — cool but tolerable, Halloweekends in full swing on weekends, brilliant fall colour around the lake.
November to April is the off-season; Cedar Point is closed, the islands are quiet, and most lakefront restaurants run shortened schedules. Sandusky does have some winter charm and is uncrowded — but the headline attractions are not in operation.