Rehoboth Beach has a clear high season (late June through Labor Day in early September) when the town operates at full capacity and prices reflect it. Accommodation is significantly more expensive in summer, and the most popular restaurants require advance reservations. The boardwalk and beach are crowded; parking is scarce and expensive.
The shoulder seasons — May through mid-June and September through mid-October — offer the best combination of decent weather, lower prices, and manageable crowds. September in particular is one of the better months on the Delaware coast: warm enough for swimming, quiet enough to get a table without booking a week ahead.
The off-season (November through April) sees the town's population drop sharply and many seasonal businesses close. A core of year-round restaurants and bars stays open, and the beach itself — grey and windswept — has its own appeal. This is the time of year when you understand why Washington's political class settled on Rehoboth: it's a place people come back to regardless of the weather.