WeekDay algorithm

function getWeekDay(nY, nM, nD)
  if not nD then
    return false, "getWeekDay(year, month, date)"
  end

  local a = math.floor((14 - nM) / 12)
  local y = nY - a
  local m = nM + 12 * a - 2

  local d = (nD + nY + math.floor(nY / 4) - math.floor(nY / 100) + math.floor(nY / 400) + math.floor((31 * m) / 12)) % 7

  return d
end