Bethany Beach is a seasonal resort. The peak season runs from Memorial Day (late May) through Labor Day (early September), when the beaches are busy, all the restaurants and shops are open, and weekend traffic on Route 1 can be brutal. July and August are the most crowded months.
The shoulder seasons are increasingly attractive: late May and June offer warm weather before the crowds peak, and September is arguably the best month to visit — the water is at its warmest from the summer's heat, the beaches are quieter, and the prices drop significantly. Many seasonal businesses stay open through October.
From November through April, Bethany is effectively a ghost town. A few year-round residents keep a handful of places open, but this is not a place for an off-season beach trip unless you specifically want long, empty beaches and closed restaurants.