Date popup select wrap
You're going to need to override an existing theme function in order alter the HTML output for the date popups. You can override a theme function either from within your template.php file within your theme, or from within a custom module.

Whenever I need to identify the correct theme function to override in a hurry, I use a module called Devel Themer. It allows you to click on any on page element, and provides you with a great break down of all the theme functions and variables associated with the element you're trying to override. 

Hope this helps.

EDIT: including theme function link

The theme function you need to override is theme_select