Why are UK bank holidays always on a Monday except Good Friday, Christmas Day and Boxing Day?
I ask, because this year, May Day Bank Holiday is next week on Monday when there is a Friday between 1st May and May Day Bank Holiday.
Some surprises for you:
What is a bank holiday was governed by the Bank Holidays Act 1871 (and several subsequent Extensions Acts) until the enactment of the Banking and Financial Dealings Act 1971 a century later, which repealed and replaced it. This Act defines what a bank holiday is, contains a schedule of the default bank holidays, and empowers the monarch (except in Northern Ireland where it is the Secretary of State) to modify the dates for a given year or to add add one-off dates to this list at whim.
This Act is the source of the first surprise. Good Friday and Christmas Day are not bank holidays, despite what you may see on many lists of U.K. bank holidays. The Act defines what a bank holiday is, and it does so in terms of a bank holiday being as free of compulsions to do banking stuff as Good Friday and Christmas Day are. Good Friday and Christmas Day were special for banking, like Sunday was, long before the idea of bank holidays was invented. They are not listed in the Act as bank holidays. Rather, bank holidays are defined to be further additional days that are like Good Friday and Christmas Day already were.
In the lists of bank holidays in Schedule 1 one can see the second and third surprises.
They are different for England+Wales, Scotland, and Northern Ireland. Only 3/5 of the England+Wales bank holidays are defined to be Mondays, the other 2/5 are calendar days. Indeed, the Act explicitly spells out that these calendar days are not always Mondays, by making provisions for what should happen if they are Sundays. (Hence only one of the 2/5 can actually occur in any given year.) It's furthermore a straight 3/6 and 3/6 split for Northern Ireland; and 2/7 Mondays, 1/7 Fridays, and 4/7 calendar days for Scotland.
This is not always Mondays. It is not even mostly Mondays for Scotland and Northern Ireland.
(Yes, the schedule lists Good Friday and Christmas Day for Scotland. Scotland had different legal traditions, meaning that these days did have to be explicitly made bank holidays. This is a flaw in the legislation, a circular definition of what a bank holiday is in Scotland, that seems to have been overlooked.)
This schedule, with only some of the bank holidays always being Mondays, is what you get unless the monarch proclaims otherwise. Since 1974, the monarch has regularly added some additional days, year after year; but, similarly, they are not always Mondays.
This convention is simply harmonizing the lists somewhat across the countries. It makes 4/7 Mondays and 3/7 calendar days (only 2 of which can ever occur in any given year) in England+Wales; and 3/8 Mondays, 1/8 Fridays, and 4/8 calendar days in Scotland.
If you read the 2018 Proclamation for 2019 you will find that it only adds these conventional additions, and makes no mention of the days that are already bank holidays per the Act, nor any mention of the days that are not bank holidays at all (except in Scotland). Contrary to the claim by St George International given in another answer, some days are not legally bank holidays, bank holidays are defined by statute, and only modifications and additions are proclaimed, albeit that it has been (at minimum) the same two added days every year for almost half a century now.
Maybe by the 150th anniversary of the original Act in 2021 someone will think to bring these long-standing conventions into statute, and fix the circular definition bug for Scotland at the same time.
From St George International:
Why are Bank Holidays on a Monday?
Each year, the dates of Bank Holidays are stated by royal proclamation. This means that HRH The Queen tells the UK government in an official letter which days will be national holidays in the coming year. This is tradition, but it means that the 'moveable holidays' around Easter will always be put on a Monday so that no holiday is lost, which happens in other countries around the world when a national holiday date happens on a Saturday or Sunday that year.