You're seeing just the function day_of_week, go back to Cldr.Calendar.Julian module for more information.
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day_of_week(year, month, day, atom)

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day_of_week(year(), month(), day(), 1..7 | :default) ::
  {Calendar.day_of_week(), first_day_of_week :: non_neg_integer(),
   last_day_of_week :: non_neg_integer()}

Calculates the day of the week from the given year, month, and day. It is an integer from 1 to 7, where 1 is Monday and 7 is Sunday.