After some problems solved, I got a tricky stuff. I am using an overlay plugin for JQuery called prettyPhoto.
Just wanna make an overlay with a form. The form is hid
Your overlay plugin 'prettyPhoto' create a new DOM everytime it is triggered, so the 'focus' event that datepicker is bounded to, does not exist on the DOM element in your lightbox.
Here is one person who had the same problem:
jQuery live() failing with jQuery UI datepicker
You'll probably need something like this:
$('#overlayData').live('click', function (){
$(this).datepicker({dateFormat: 'dd/mm/yy'});
});