Days are calculated to include weekends but not Bank Holidays if you are looking at the Payment and Adjudication timetable. So if you have to do something like issue a Payless Notice not later than 5 days before, say, Tuesday 27th April 2021 then you would need to issue it today; as that is not less than 5 days before Tuesday 27th April 2021. The active days would be 22nd, 23rd, 24th, 25th and 26th April.
If Monday 26th was a Bank Holiday it would not count so you would have to issue a day earlier. It gets very complicated around Christmas.
I always err on the side of caution as if you get it wrong then you can end up in a smash and grab situation.