If you’re going to commemorate the birth and death of a messiah, Christmas and Easter (Good Friday and Easter Monday (?) are holidays in the UK. Learned that when I worked at Reuters in New York.
New Years Eve / Day just because it’s fun.
If you’re going to send a document to a King about why his colonists have been fighting your soldiers for well over a year, July 4th. I dunno if when the conflict started Independence was the goal, yet in 1776 independence it is. Almost every country has either an Independence day or Constitution Day (Norway at least)
If MLK jr still has a holiday (I recall when it was created it was swapped with either President’s Day or Washington’s birthday and also isn’t called that in some states) he deserves it.
Not generally a day off, yet if people learn that trees are good to have, Arbor Day.
Columbus never even knew of mainland Americas so any day commemorating him is not only manufactured, yet undeserved.
A day off only in Ireland, AFAIK (and they may have changed that since I left - or just allowed pubs to open), yet I dunno what in particular March 17th means.
Enforcing the law of the USA - even if you’re just a stubborn territory like Texas long after the 14th amendment and surrender of Lee at Appomattox, you’re just looking for an excuse for a June Holiday with a clever portmanteau name.
In the UK there are the creatively named Early May Bank Holiday and Late May Bank Holiday and a similar pair two weeks apart in August.
Memorial and Veteran’s day (Not "The USA won WWI all by itself Day like Trump did with VE-day and WWII) are commemorative and deserved.
They’ve been doing Halloween in Ireland and the UK more and more, yet that’s appropriating a Druid day or All Soul’s Eve (as Christmas sort of is), take your pick. If Moses hadn’t dropped the 11-15th commandments, one of them might have demanded giving out candy.