Originally Posted by
UKtravelbear
it is something that the travel trade could make clearer. BA Holidays give you a date by which you need to pay the balance of your holiday by which is 3 weeks before departure but they actually give you the specific date you don’t have to guess. And that 3 weeks doesn’t include the day of departure.
A payment deadline is something in a different league, though; miss the payment deadline, and you presumably lose both your deposit, and the chance to go on holiday for the agreed price! It's a deadline by which something
must happen (if not, something else happens as a result); in the OP's case, it's not a requirement that they must change the ticket, or react in any way before any particular time limit.