I mean, in the US we celebrate Independence Day, Presidents Day, Veterans and Memorial day. Why only have one ?
That's not how Matalas described it in the linked interview: It is the 250th anniversary celebrating when the NX-01 went out into space and the de-facto birth of modern Starfleet, and you will hear more about it. Starfleet didn’t really solidify until the NX-01 went out – this was the first ship with a multi-species crew and its voyage was pivotal to the founding of the Federation. It was essentially the birth of Starfleet as we know it.
Well, there's a reason I analogized Frontier Day to Independence Day as commemorating something that's symbolically meaningful while practically irrelevant.
... or something they took the time to flesh out from the multiple episodes of ENTERPRISE that actually depicted that time period. The launch of the NX-01 was a pretty big deal at the time and though he wasn't in any way perfect, Captain Archer became the de facto representative of Earth and the one who essentially brought about the birth of the Federation.
... as the many years of produced Trek shows and movies have proven time and time again on this BBS alone.
Black Friday has basically become an international event, even in countries which don't celebrate the American Thanksgiving.
It clearly wasn't because it's a plot point of the season. The attacks are leading up to Frontier Day.
Black Friday isn't a holiday. It's a commercial promotion, basically. Yes, both Canada and the UK have adopted it, but it isn't as big in either place as it is in the US. Other countries do not, however, celebrate US holidays proper. They may overlap their own holidays, e.g. President's Day is Monday in the US, and Canada -- minus Quebec -- has Family Day. But it would make no sense for Canada to celebrate the birth of George Washington....