First of all, I will nitpick. It’s the “observance” of holidays, not “observation”. The words have much different meanings.
Most legal holidays in the U.S. are moved to a Monday to give people a three-day weekend because employers like it that way. When you have a midweek holiday, people tend to take off the days before and after and it really cuts down on productivity.
If the holiday is Monday, like Columbus Day tomorrow (I will be doing my best tomorrow to explore something in memory of Columbus. That and doing my laundry), most people will just come back to work on Tuesday and everyone is no worse for wear.
But some holidays just can’t be moved around. Obviously, there’s New Year’s Day. Christmas is another one. If those holidays fall on a weekend, some employers will give their workers a day off on the Friday before or the Monday after the holiday for contractual purposes, or to avoid being regarded as Ebenezer Scrooge.
Two other holidays that are not moved around are Independence Day, a date that is so well-known and so important to nearly everyone in the U.S., that moving it would be foolhardy.
Veterans Day is an interesting case. For a three or four year period in the early 1970s, Congress tried to move Veterans Day from November 11 to the last Monday in October. Some thought it was too close to Thanksgiving and it was causing trouble because it was a midweek holiday.
Veterans groups were, to say the least, pissed off about the change. November 11 is a very important day to veterans, at least to those who fought in World War I who were still alive in much larger numbers when the change was made briefly, put forth a big lobbying effort to get the day switched back to November 11, where it is has remained locked in place.
This year Veterans Day falls on a Sunday. My workplace is open Sundays, but will close that Sunday for the holiday and close on Monday, so it can fulfill our contracted number of holidays, Sunday not being a regular work day. This is good for me, but probably sucks for our customers.
I can’t speak for other countries, but I know that Canada keeps some holidays fixed, such as Canada Day (July 1), but moves around to Monday holidays like Victoria Day…