updating DOM through ajax call back with Javascript

I'm working on a calendar that will allow users to store events on a given date. when I update the calendar each new month I create new nodes and assign them an id so I will be able to append something to them later like so:

 var i = 1;
  for(var w in weeks){
    var days = weeks[w].getDates();
    // days contains normal JavaScript Date objects.

   // alert("Week starting on "+days[0]);
    var which_week = "week"+i;
    i++;
    for(var d in days){     
      console.log(days[d].toISOString());
      var tr = document.getElementById(which_week);
      if(days[d].getMonth()==month){

        var newDay = document.createElement("div");
        newDay.appendChild(document.createTextNode(days[d].getDate()));
        //alert(newDay.data);
        newDay.setAttribute("id", newDay.lastChild.data);
        $(tr).append('<td><a class="linky" href="#">'+newDay.lastChild.data+'</a></td>');
      }
      else{
        $(tr).append('<td class="disabledCell"><a class="linky disabledLink" href="#">'+days[d].getDate()+'</a></td>');
      }
    }
  }
  getEvents();
}

I modify the nodes:

function ajaxEventCallback(event){
  var data = event.target.responseText;
  data = JSON.parse(event.target.responseText);
  for  (var i = 0; i < data.length; i++) 
    //alert("event: " + data[i].title);
    //
    var dayOfEvent= data[i].day;
    document.getElementById(dayOfEvent).appendChild(data[i].title);

}

I get a "cannot read property append child of null" error on the last line of code. I think it's a scoping issue but I don't know where to begin solving it.