calendar.js

/*
(4) 2018 M05
Mo  Di  Mi  Do  Fr  Sa  So

     1   2   3   4   5   6
 7   8   9  10  11  12  13
14  15  16  17  18  19  20
21  22  23  24  25  26  27
28  29  30  31

(5) 2018 M06
Mo  Di  Mi  Do  Fr  Sa  So

                 1   2   3
 4   5   6   7   8   9  10
11  12  13  14  15  16  17
18  19  20  21  22  23  24
25  26  27  28  29  30
*/

function pad (str) {
  return (' ' + str).slice(-2)
}

function displayMonth (num) {
  var date = new Date()
  var y = date.getFullYear()
  var m = date.getMonth()
  var firstDay = new Date(Date.UTC(y, m + num, 1))
  var lastDay = new Date(Date.UTC(y, m + 1 + num, 0))

  var w = firstDay.getUTCDay() - 1 // der WochenTag
  var i
  var d = 1 // der Tag
  var text = ''
  text += '(' + firstDay.getMonth() + ') ' + firstDay.toLocaleDateString('de-DE', { year: 'numeric', month: 'long' }) + '\n' //  (4) ‎Mai‎ ‎2018
  text += 'Mo  Di  Mi  Do  Fr  Sa  So\n'
  for (i = 0; i < lastDay.getUTCDate() + firstDay.getUTCDay() - 1; i++) {
    // console.log('i', i, 'w', w, 'd', d)
    if (i % 7) {
      text += '  ' // zwischen den Daten
    } else {
      text += '\n' // nach einer Woche
    }

    if (i === w) {
      text += pad(d)
      w++
      d++
    } else {
      text += '  ' // der Platzhalter für den vergangenden Monat
    }
  }
  return text
}

console.log(displayMonth(0))
console.log()
console.log(displayMonth(1))