dayOfWeek = new Array(
    "Sun",
    "Mon",
    "Tue",
    "Wed",
    "Thu",
    "Fri",
    "Sat");

function updateDay(id, m) {
    var myMonth = document.getElementById(id +"-"+ m +"month").value;
    var myDate = document.getElementById(id +"-"+ m +"date").value;
    var myYear = document.getElementById(id +"-"+ m +"year").value;
    var dateObj = new Date(Date.parse(myMonth +" "+ myDate +", "+ myYear));
    document.getElementById(id +"-"+ m +"day").innerHTML = dayOfWeek[dateObj.getDay()];
}