I literally saw this at my first work 20 years ago. You generate most by script and just add those few odd public holidays manually, much easier to have a person spend 5 mins once every year rather than some smart script with 20 not-so-well-handled corner cases and 7 bugs.
Oh goodness no, we don't generate ANY of it, we require the HR dept to own the calendar data. They can copy from the previous years calendar if they want as a button in the UI, but I honestly have no idea if they use that button.
Like I mentioned in a diff. comment, we generate all the official calendars of the org from this data. We print them, we generate .ics, etc.
