Using pure numbers as id's is what was causing the issue. I would suggest you change them to
<div id="zip60538">
Welcome to Montgomery
</div>
<div id="zip60504">
<h1>Welcome to Aurora</h1>
</div>
In the html (as well as in the css and the js as can be seen in the linked fiddle)
This will allow them to be properly referenced in the DOM
edit
If you had a lot of these to handle, I would probably wrap the html area in a div to localize it and then use an array to store the accepted zip codes, and make use of both of those approaches in the click event
$(document).ready(function(){
var zipCodes = [60538,60504];
$("#buttontest").click(function(){
var zipIndex = zipCodes.indexOf(parseInt($("#full_day").val()));
$("#zipMessage > div").hide("fast");
$("#zip"+zipCodes[zipIndex]).show("fast");
});
});