Étretat is at its best late April to early June and again in September. The weather is generally fine, the gardens are open, the cliffs are walkable, and the crowds are bearable. July and August are the peak — the town fills, parking gets hard, and the beach can disappear under bodies. Restaurants take more advance booking than they would in a city.
Winter is wild and atmospheric but limited: many of the smaller shops, museums, and restaurants close from November to March, and the cliff paths are slippery. The chalk and the spray under a winter storm are beautiful, though, and you may have the cliff-tops to yourself.
Whatever season you visit, check the tide tables before you go. The arches and the base of the cliffs are only safely accessible around low tide, and the difference here is dramatic — over 8 metres at spring tides. Many of the best photographs are taken at low water.