index.js

let newDateFunction = new Date()
function randerDate(){
  newDateFunction.setDate(1)
  let day = newDateFunction.getDay()

  let currentDate = new Date(
      newDateFunction.getFullYear(),
      newDateFunction.getMonth() + 1,0
  ).getDate() // to get the last date of current month


  let prevDate = new Date(
      newDateFunction.getFullYear(),
      newDateFunction.getMonth(), 0
  ).getDate() // to get the last date of previous month

  let addNext = new Date(newDateFunction.getFullYear(), 
  newDateFunction.getMonth() + 2, 0).getDate() + 7;

  console.log(currentDate, prevDate, addNext);

  let month = newDateFunction.getMonth()
  let year = newDateFunction.getFullYear()
  let monthArr = ['January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December']
  document.getElementById("month").innerHTML = monthArr[month] + " - " + year

  let today = new Date()
  let weekDay = today.getDay()
  document.getElementById("date").innerHTML = today.toDateString()
  document.querySelector(`.week :nth-child(${weekDay + 1})`).classList.add("active")



  let DATES = ""

  for(let x = day; x > 0; x--){
      DATES += "<div class='prev'>" + (prevDate - x + 1) + "</div>"
  }

  for(let i = 1; i <= currentDate; i++){
      if(i === today.getDate() && newDateFunction.getMonth() == today.getMonth() && newDateFunction.getFullYear() === today.getFullYear()){
          DATES += "<div class='today'>" + i + "</div>"
      }
      else{
          DATES += "<div>" + i + "</div>"
      }
  }

  for(let k = 1; k <= addNext; k++){
      if(k <= 1){
          DATES += "<div class='next'>" + k + "</div>"
      }
      else{
          break;
      }
  }

  document.querySelector('.dates').innerHTML = DATES
}

function moveDate(para){

  if(para == 'prev'){
      newDateFunction.setMonth(newDateFunction.getMonth() - 1)
  }

  else if(para == 'next'){
      newDateFunction.setMonth(newDateFunction.getMonth() + 1)
  }

  randerDate()
}

var modal = document.getElementById("myModal");
    var btn = document.getElementById("submit-btn-amenity");
    var span = document.getElementsByClassName("close")[0];

    btn.onclick = function() {
        modal.style.display = "block";
    }

    span.onclick = function() {
        modal.style.display = "none";
    }

    window.onclick = function(event) {
        if (event.target == modal) {
            modal.style.display = "none";
        }
    }