Intrigued by how this works days or weeks before (except in the case of an aircraft swap being known in advance) given that the extent to which passengers will turn up in numbers greater than originally estimated is generally only known at check in stage, probably <24h before departure.

Still, even communication on the morning of the flight beats finding out at the airport (Gold standard for poor service goes to airBerlin, who somehow managed not to inform me that they'd actually cancelled my entire flight days before until I turned up for it in Greece with a handful of other bemused holidaymakers, most of whom unlike me hadn't booked direct. I'd have accepted a much less expensive and more flexible date for an alternative flight though.)