Maxime Tremblay has already described here how to customize the datepicker.

He had to deal with missing classes after changing options. I found this week a different way to customize the datepicker without loosing classes added by APEX: Simply by setting datepicker defaults on page load before any datepicker is initialized.

In my case I wanted to enable the button panel which shows two buttons - one to go to the current month which is called Today and a second one to close the datepicker which is called Done I only wanted the today button, so I had to provide beside the JavaScript or dynamic action some CSS to hide the second one:

apex.jQuery(document).ready(function(){
  $.datepicker.setDefaults({ 'showButtonPanel': true });
});
body .ui-datepicker-close { display: none !important; }

jQuery datepicker today button

Just for reference: The jQuery UI datepicker widget can be tested here. Be careful to pick the correct version of the jQuery UI API docs for your specific APEX version.

Happy coding :-)
Ottmar