Elixir v1.12.0

Calendar.day_of_week

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

day_of_week(year, month, day, starting_on)

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Specs

day_of_week(year(), month(), day(), starting_on :: :default | atom()) ::
  {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.

The starting_on represents the starting day of the week. All calendars must support at least the :default value. They may also support other values representing their days of the week.