function getDate(objDate) {
  // (objDate)YYYY-MM-DD 타입으로 입력하세요.
  var arrDate = objDate.split("-");

  //입력일의 요일.( 0:일요일, 1:월요일, 2:화요일, 3:수요일, 4:목요일, 5:금요일, 6:토요일 )
  thisDate =new Date(arrDate[0],arrDate[1]-1,arrDate[2]);
  var thisWeek = thisDate.getDay();

  //입력일의  마지막일자
  lastDate =new Date(arrDate[0],arrDate[1],0);
  last2Date =new Date(arrDate[0],arrDate[1]-1,1);
  var lastDays = lastDate.getDate();
        var monthSWeek = last2Date.getDay();
  //alert(monthSWeek);
  var nWeek = parseInt((parseInt(arrDate[2]) + monthSWeek - 2)/7)+1;

  //alertStr
  var alertStr = "//입력일자 : " + arrDate[2];
  alertStr+= "//입력요일 : " + thisWeek;
  alertStr+= "//마지막일 : " + lastDays;
  alertStr+= "//몇주차? : " + nWeek;
  //alertStr+= "//입력요일 : " + thisWeek;
  alert(alertStr);
 }

[출처] javascript, 입력년월의 일자,입력요일,마지막일,월의 몇번째주|작성자 야옹메리

http://pcwithme.blog.me/130090016619