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

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week_of_month(year(), month(), day()) ::
  {pos_integer(), pos_integer()} | {:error, :not_defined}

Calculates the week of the year from the given year, month, and day. It is an integer from 1 to 53.