In the UK a public holiday (such as Christmas Day) falling at a weekend is given on the next available working day, so the Monday (whether 25th is a Saturday or Sunday). Boxing Day on 26th December may move to the following Monday, or the Tuesday if Christmas has already moved to the Monday. Our UK-specific “Bank Holidays” are always on a Monday (eg first Monday in May, last Monday in August); Easter holidays always Good Friday and Easter Monday, so none of these ever have to move.
In other countries such as the US (I believe), public holidays based on a fixed date move to the nearest available working day, so Saturday > Friday, Sunday > Monday. Again, some are on specific weekdays anyway so never move, such as President’s Day or Thanksgiving.
In many European countries they appear to have loads of public holidays for many saint’s days, but in lots of cases such as in Germany, these are always on a fixed date and do not move, so each year some are lost and some are holidays, so on average they actually end up with a similar number of free days to the UK.
Outlook does not even attempt to understand such complex rules, each holiday is simply an event in the calendar, so you can either create a calendar file with the original festivals moved (eg Christmas Day on Monday December 26th) which can look odd, or add ‘lieu’ days instead, which is what I tend to do so people don’t think their calendar is broken. I use Excel to take the original dates and then create lieu dates as well where needed, and parse all this together ready to export to a text file.

Also I prefer UK spelling to US, so “Labor” becomes “Labour” etc.