Summer (June to August) is high season, especially on Sylt, where prices and visitor numbers peak in July and August. Föhr and Amrum are slightly quieter. The weather is mild but changeable — wind is constant, sun is not guaranteed, and a jumper is always useful even in July.
May and September are excellent shoulder months: the beaches are usable, the crowds thinner, and the light on the tidal flats is often better for walking and birdwatching. Spring migration (April–May) and autumn migration (September–October) bring enormous numbers of wading birds to the Wadden Sea, and birdwatching at these times is spectacular.
Winter is dramatic but spare. Storm surges occur between November and February, occasionally flooding the halligen. If you are on the mainland coast during a storm surge, watching the sea come in is an experience in itself.