function lastDayOfMonth(y,m,dy) {
 var  days = {sun:0,mon:1,tue:2,wed:3,thu:4,fri:5,sat:6}
     ,dat = new Date(y+'/'+m+'/1')
     ,currentmonth = m
     ,firstday = false;
  while (currentmonth === m){
    firstday = dat.getDay() === days[dy] || firstday;
    dat.setDate(dat.getDate()+(firstday ? 7 : 1));
    currentmonth = dat.getMonth()+1 ;
  }
  dat.setDate(dat.getDate()-7);
  return dat;
 }
// usage 
lastDayOfMonth(2012,2,'tue'); //=>Tue Feb 28 2012 00:00:00 GMT+0100
lastDayOfMonth(1943,5,'fri'); //=>Fri May 28 1943 00:00:00 GMT+0200