From context you might know if it seems like they know how to use the phrase, but I always struggle to understand these quirks, perhaps because I heard these terms as an adult and haven’t used them much myself, or been exposed to them and the context enough to immerse myself in the colloquial usage by diffusion.
This is close to weird constructions like “x is deceptively y” like “the dog is deceptively large” which, without already knowing the size of the dog, makes me feel like a dunce because I don’t know while not giving me enough specificity to know if the the largeness is what is deceptive or just the perception of the largeness. It’s a syntactical tarpit.
> I use "<day> week" in conversation, but I'd say it's falling out of favour. I mostly use it with my parents.
I am a native English (US) speaker, and I think it’s a British English thing perhaps, as I heard it all the time in Australia, along with other week-related terms like fortnight.
