Gmail – How to set up auto-reply for each email from the same sender

In short, no you can't. You can use the 'automatic vacation' responder found on the first page of settings within GMail to set a specific date, however you're going to have to do that at the end of each week.

More here - http://support.google.com/mail/bin/answer.py?hl=en&answer=25922

One can include an HTML message in reply by using the reply(body, options) syntax, instead of the reply(body) syntax used above. Plain text body must still be provided for those clients who do not render HTML in messages.

The relevant parts are

var message = "This is my day off.";
var htmlMessage = "This is <strong>my day off</strong>.";
//  later 
  threads[i].reply(message, {htmlBody: htmlMessage});

The complete script becomes

function autoReply() {
  var interval = 5;          //  if the script runs every 5 minutes; change otherwise
  var daysOff = [4,5,6,0];   // 1=Mo, 2=Tu, 3=We, 4=Th, 5=Fr, 6=Sa, 0=Su
  var message = "This is my day off.";
  var htmlMessage = "This is <strong>my day off</strong>.";
  var date = new Date();
  var day = date.getDay();
  if (daysOff.indexOf(day) > -1) {
    var timeFrom = Math.floor(date.valueOf()/1000) - 60 * interval;
    var threads = GmailApp.search('is:inbox after:' + timeFrom);
    for (var i = 0; i < threads.length; i++) {
      threads[i].reply(message, {htmlBody: htmlMessage});
    }
  }
}