Calculate the week day in pure Lua. Only depends on the math package to floor numbers.

weekday.lua

--[[----------------------------------------
    Project:   	LibClockTST
    Author:     Arne Rantzen (Tyx)
    Created:    2020-02-16
    License:    GPL-3.0
----------------------------------------]]--

--- Get the day of the week by the guassian algorithm
--@param table should be containing three ints: year, month, day
--@return weekday number after ISO norm from 1 = Monday to 7 = Sunday
--@see https://en.wikipedia.org/wiki/Determination_of_the_day_of_the_week#Gauss's_algorithm
local function GetGaussWeekDay(table)
  assert(type(table.year) == "number" and type(table.month) == "number" and type(table.day) == "number",
		"Please provide a table like {year = 2020, month = 2, number = 16}")

	local function GetDaysFromZero(y)
		local leaps = math.floor(y / 4) - math.floor( y / 100) + math.floor(y / 400)
		return 365 * y + leaps
	end

	local mm = (table.month % 12 + 9) % 12 -- End is February (11), start is March (0)
	local dd = GetDaysFromZero(table.year + math.floor(table.month / 12) - math.floor(mm / 10))
	local totalDays = dd + math.floor((mm*306 + 5)/10) + table.day - 307
  return (totalDays) % 7 + 1 -- shift monday to 1st and sunday to 7th
end

--- Get the day of the week by the disperate algorithm
--@param table should be containing three ints: year, month, day
--@return weekday number after ISO norm from 1 = Monday to 7 = Sunday
--@see https://en.wikipedia.org/wiki/Determination_of_the_day_of_the_week#Disparate_variation
local function GetDisparateWeekDay(table)
  assert(type(table.year) == "number" and type(table.month) == "number" and type(table.day) == "number",
		"Please provide a table like {year = 2020, month = 2, number = 16}")
  if table.month < 3 then
    table.year = table.year - 1
  end

  local y = table.year
  local c = 0
  if tostring(y):len() > 2 then
    c = tonumber(tostring(y):sub(1, -3))
    y = tonumber(tostring(y):sub(-2))
  end
  local d = table.day
  local m = (table.month % 12 + 9) % 12 + 1
  local w = (d + math.floor(2.6 * m - 0.2) + y + math.floor(y / 4) + math.floor(c / 4) - 2 * c) % 7
  if w < 0 then
    w = w + 7
  end
  return (w - 1) % 7 + 1 -- shift monday to 1st and sunday to 7th
end