July and August are high season — hot, sunny, expensive, and crowded. The surf at Côte des Basques and Grande Plage is busy but good, the evenings are long and warm, and the town is fully alive. Book accommodation well in advance.
June and September are the best months: warm enough for the beach, quieter than peak season, and more affordable. September surf is often better than summer, as Atlantic swells build and the crowds thin. The Biarritz Surf Festival (usually early July) brings professionals and thousands of spectators — avoid it if you dislike crowds, seek it out if you want to watch world-class surfing.
October to March is the quiet season. The town empties out, many restaurants close or reduce hours, and the weather is Atlantic — grey, wet, and occasionally stormy. But the storms produce the biggest surf of the year (the famous winter swells at Belharra, 3 kilometres offshore, can reach 15 metres or more), and hotel prices collapse to a fraction of summer rates. Bayonne's Fêtes de Bayonne, a major Basque festival with bull-running and street parties, happens in late July.