Use Zeller's Congruence to determine the day of the week, given
# year, month and day as integers in the conventional way.
# Emit 0 for Saturday, 1 for Sunday, etc.
#
def day_of_week(year; month; day):
  if month == 1 or month == 2 then
    [month + 12, year - 1]
  else
    [month, year]
  end
  | day + (13*(.[0] + 1)/5|floor)
    +  (.[1]%100)       + ((.[1]%100)/4|floor)
    +  (.[1]/400|floor) - 2*(.[1]/100|floor)
  | . % 7
;