Happy thanksgiving everyone.
I knew it was a "humour" question, so didn't read it properly at first -- didn't look at the code) so didn't notice the dateadd. But then I read "Yellow Thursday" and thought "what on earth could Thanksgiving have to do with that ... surely it would be suicide to call it that in many places in the USA" and decided to look at the code. So that left me with Black Friday or Green Friday. I reckoned that it was unlikely that Steve had decided to be sort of politically correct and go with the tiny minority who call it Green Friday and try to persuade people to do Green things or charitable things rather than joining in the stampede, so I picked Black.
Interesting satement in the explanation that this is when many retailers start their Christmas season - it seems that American retailers have more of a sense than English ones: in most of England the "special Christmas offers" start appearing some time in August. I don't know whether this stupidity has penetrated Wales yet, and it hasn't reached the parts of Scotland that I get to in August, but in England it appears to be pretty well universal - not in the sense that every retailer does it, but in the sense that at least one retailer in every shopping mall or on every shopping street does it.