September is when the swell window opens again. The first autumn lows track far enough south to send north-westerly swell into the Bay of Biscay, and a coast that has had nothing to work with since April starts breaking properly again, on water that is still at 21°C.
The corner is what makes this month specific. At the border the coast swings east, so a wind that is onshore on the open French sand comes off the land here: on the same morning, the Landes can be a mess of whitewater while the Basque beaches forty minutes south are glassy and head high. September is the first month of the year when there is enough swell for that difference to matter.
The Spanish holidays end on the first of the month, and the beaches empty over one weekend, which is the other half of what September is for.
