Cldr.Calendar.month_of_year

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

month_of_year(year, month, day)

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Specs

month_of_year(
  year :: Calendar.year(),
  month :: Calendar.month() | week(),
  day :: Calendar.day()
) :: Calendar.month()

Returns the month for a given year, month or week, and day for a a calendar.

The month_of_year is calculated based upon the calendar configuration.