为什么星期出不来

//通过javascript的日期对象来得到当前的日期，并输出。

  var mydate =new Date();

  var weekday = ["星期日","星期一","星期二","星期三","星期四","星期五","星期六"];

  document.write(

      mydate.getFullYear() + "年" +( mydate.getMonth()+1 )+ "月" + mydate.getDate() + "日" + "" + weekday[mydate.getDate()] +"<br/>");