From type/Dateish

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method day-of-week(Date:D: --> Int:D)

Returns the day of the week, where 1 is Monday, 2 is Tuesday and Sunday is 7.

say Date.new('2015-12-31').day-of-week;                                  # OUTPUT: «4␤» 
say DateTime.new(date => Date.new('2015-12-24'), hour => 1).day-of-week; # OUTPUT: «4␤»