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The day is pretty irrelevant in finance as it is too granular, but the problem of converting from US week numbers comes up.

Its also used in manufacturing, tire date codes for instance are week.year or year.week

Also used in supermarkets for planning. It's handy to see how much of product was purchased in week 34, spanning back a few years.

Used in all of retail. "Assortment Planning"

As far as I have seen finance prefers months and quarters, not being able to tell which year a month is in is a non-starter.

They do sometimes pretend each month is 30 days though.