This is true for all public holidays in the UK, there’s a (usually) fixed number of public holidays but the dates are flexible.
They’re also included in the minimum 28 days paid time off too, meaning if you’re a full time worker and have to work on a bank holiday your employer is legally required to offer an extra day off somewhere else instead, either a fixed date or added to your holiday allowance. Conversely, the “extra” day off you get when a monarch keels over may be subtracted from your holiday allowance for the year. This is also why my employer is allowed to follow English bank holidays despite having next to no presence in England; the number is fixed but the dates are not.
I can’t think of a scenario where you ask someone to defend why they don’t drink but they are the one that is insecure and judgemental. There’s plenty of situations where people drink when they shouldn’t (eg driving) but it’s not quite so common to be in a situation where you must drink but don’t.