new Date(date.getFullYear(), (date.getMonth()+1),0).getDate()

 var weekday=new Array(7);
 weekday[0]="星期日" ;
 weekday[1]="星期一";
 weekday[2]="星期二";
 weekday[3]="星期三";
 weekday[4]="星期四";
 weekday[5]="星期五";
 weekday[6]="星期六";
 alert("本月第一天是 " + weekday[date.getDay()]);