You're seeing just the callback week_of_year, go back to Cldr.Calendar module for more information.
Link to this callback

week_of_year(year, month, day)

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  year :: Calendar.year(),
  month :: Calendar.month() | week(),
  day ::
) :: {Calendar.year(), Calendar.week()} | {:error, :not_defined}

Returns a tuple of {year, week_in_year} for a given year, month or week, and day for a a calendar.

The week_in_year is calculated based upon the calendar configuration.