I had a recent experience that might help.

I had a long weekend booked to Dubai departing on 22nd January, I believe the balance due date was 26th December 2019 but with being away over Christmas and ignoring my emails I forgot and missed the payment date.

BA Holidays text my mobile and then called me on Friday 3rd January to say the balance needed to be cleared by the close of play on 4th January or the holiday would be cancelled. I paid it that day but based on my experience you get a week after the due date and they will call you. Like others have said I'm sure they will extended it for a week formally if you call them.