JavaScript Addition Date Functions
function getEndOfWeek() {
    var today = new Date();
    var weekDay = today.getDay();
    // if you want the week to start on Monday instead of Sunday uncomment the code below
    //weekDay -= 1;
    //if(weekDay < 0) {
    //    weekDay += 7;
    //}
    var daysToAdd = 6 - weekDay;
    var newDate = new Date(today.getTime() + daysToAdd *24*60*60*1000);
    var month = newDate.getMonth() + 1;
    var day = newDate.getDate();
    var year = newDate.getFullYear();
    var formatedDate = month + "/" + day + "/" + year;
    return formatedDate;
}

$(function() {
    $("#TheDate").html(getEndOfWeek());
});

The week ends on <span id="TheDate"></span>.

weekDay -= 1;
if(weekDay < 0) {
    weekDay += 7;
}

var day = 1000*60*60*24
, nextSunday = new Date(+new Date() + day*(7-((0|(+new Date()/day)%7-3)||7)));

alert(
    (101+nextSunday.getMonth()).toString().substr(1) + '/' +
    (100+nextSunday.getDate()).toString().substr(1) + '/' +
    nextSunday.getFullYear()
)