А вот за такой вариант решения, например на работе, ничего плохого не скажут? Не сделают? ))
let mass = ['понедельник', 'вторник','среда','четверг','пятница','суббота','воскресенье']
let monday = [1,8,15,22,29,36]
let Tuesday = [2,9,16,23,30,37]
let Wednesday = [3,10,17,24,31]
let Thursday = [4,11,18,25,32]
let Friday = [5,12,19,26,33]
let Saturday = [6,13,20,27,34]
let Sunday = [7,14,21,28,35]
let massbig = [monday,Tuesday,Wednesday,Thursday,Friday,Saturday,Sunday]
let getDayOfWeek = function(dayWeek,nomer){
let who = mass.indexOf(dayWeek)+1
let summ = 0
if (nomer==1 & who==1){
return 'понедельник'}
else { summ = nomer + who - 1}
let poisk = monday.indexOf(summ)
if (poisk > 0) {let trash = massbig.indexOf(monday)
return (mass[trash])}
poisk = Tuesday.indexOf(summ)
if (poisk > 0) {trash = massbig.indexOf(Tuesday)
return (mass[trash])}
poisk = Wednesday.indexOf(summ)
if (poisk > 0) {trash = massbig.indexOf(Wednesday)
return (mass[trash])}
poisk = Thursday.indexOf(summ)
if (poisk > 0) {trash = massbig.indexOf(Thursday)
return (mass[trash])}
poisk = Friday.indexOf(summ)
if (poisk > 0) {trash = massbig.indexOf(Friday)
return (mass[trash])}
poisk = Saturday.indexOf(summ)
if (poisk > 0) {trash = massbig.indexOf(Saturday)
return (mass[trash])}
poisk = Sunday.indexOf(summ)
if (poisk > 0) {trash = massbig.indexOf(Sunday)
return (mass[trash])}
}