This would be illegal in Denmark https://www.forbrugerombudsmanden.dk/hvad-gaelder/markedsfoe...
English
>If an item, for example, has cost DKK 1,000 throughout the year, and it is set up to DKK 1,500 for 6 weeks and then comes on “sale” for DKK 1,000 again, this is misleading, which is illegal.
obviously you can do stuff like set the price up for a longer period to escape being illegal but then you probably have a bunch of stock sitting around that doesn't move.
although as a general rule when I see black friday deals in Denmark I am not too thrilled at the savings, maybe because they tend to be realistic markdowns.