[retsibsi]

That's just a spicy way of saying "I am unfamiliar with this idiom". Nobody is saying you should unilaterally start using it in the US, or in any other context where nobody would understand you. They're saying that, for those who do have this idiom, it is unambiguous.

[thaumasiotes]

There's a little bit more to it than that. I am unfamiliar with the idiom, and the idiom does not appear to be grammatical English, suggesting that something has gone wrong rather than that the speaker is using a foreign vocabulary item. Most idioms don't look like word salad, but this one does.

"Friday week" would ordinarily mean a week characterized in some way by Friday, but of course there can be no such week. There could be a "Good Friday week".

[retsibsi]

Your original complaint was that the phrase is meaningless. To people who are familiar with it, it's obviously not meaningless! For those who are unfamiliar with it, I'd say the bafflingness is more feature than bug; you'll immediately know that you've encountered an unfamiliar phrase (or missed a word), rather than trying to piece it together logically and coming away with an illusion of understanding.

(Yeah, it would be even better if it just made sense transparently and unambiguously to all listeners. But that leaves us with a complaint about idioms in general, not this one in particular.)

[aspenmayer]

> "Friday week" would ordinarily mean a week characterized in some way by Friday, but of course there can be no such week. There could be a "Good Friday week".

I think there was that one or maybe two times the Catholic Church changed the day or date? I don’t know much about it but that may have resulted in a week without a Friday, which would make the next one pretty good, when it happened.