I used to laugh at the countries trying to ban/suppress "Western Culture." Now, that looks like a pretty good idea.
IIRC, the term "Black Friday" meant that the Friday after Thanksgiving was generally the day when retailers broke even for the year, and went into the "black" (meaning black ink on the ledger page instead of red.) Everything after that was profit.
As to "Trick-or-treat", you Brits can't blame us Yanks totally for that one. Wassailing, Souling, "Guising" in Scotland- all these were from your side of the pond.
Folks forget this verse of the Gloucester(shire) Wassail:
Come Butler, come fill us a bowl of the best,
Then we hope that your soul in heaven may rest.
But if you do draw us a bowl of the small,
Then down shall go butler, bowl and all.
As to "Prom", I suspect that it has its origins in the "Debutante Ball" where high-society girls who had reached marriageable age were paraded in front of potential suitors like horses at an auction.

And, since every generation of Americans has made it their goal to live like the rich of the previous generation, this custom got transplanted to the Great Unwashed Masses.

Bit of a competition to see who can afford the most elaborate dress, the finest jewelry, the fanciest hairdo, the largest limousine (or most expensive- somebody showed up at my daughter's prom in a vintage Rolls Royce.) And of course having the prom in a decorated school gymnasium is now considered tacky, so they all have to show up at the school for "walk-in" with photos taken, then they all get picked up by luxury buses and limousines and taken to dinner and dancing at a suitably expensive venue.

Lomax wrote:No, not Thanksgiving, though that's probably only a matter of time. What happens in America does tend to happen here at some stage. Trick or Treat and the High School Prom were totally unknown here when I was growing up. Now, kids look on them as a birthright.
But yesterday we had a new one. Black Friday. And I, numpty that I am, didn't know what this meant, and went to the shops in all innocence....
Oh dear.