It depends on the business-definition. Which is usually the ISO definition, but not always.

Most places I've worked, they want the first week to include 1 Jan, but to start on a Monday and end on a Friday (or Sun-Sat), so I have to calculate week-beginning even for 1 Jan.

But I have to admit, I generally prefer a persisted Calendar table over a "runtime calendar". You can't store holidays in a runtime version. My current employer gave us the 31st of December off this year, because it was a Monday and the 1st a Tuesday. Normal years, they don't do that. A workdays calculator built using a numbers table (tally table) can't easily record that extra, "not by the usual rules" paid holiday. A persisted calendar table can easily record that kind of thing. Sure, you can store a table just of holidays, and use an Except query to get those into a runtime calendar, but that's more expensive (server resources) and complicated.

With a persisted calendar, you can present it to management, via a simple UI, and they can confirm and sign-off on it. Want the 1st week of the year to begin on 30 Dec (Sunday like it just did)? Sure, no problem. Just record that in the "BusinessYear" and "WeekOfYear" columns. No tricky coding required.