This isn't that straight forward,
There is a UI Macro called show_schedule but this is just the 'decoration' that adds an icon to the form next to the field. When you click the icon it opens show_schedule.do which is a page defined in a .xml file on the file system.
To override this page you would have to obtain a copy of its XML, modify it and then save it as a new UI Page with the same name to override.
This would also be pretty bad practice.
Instead you should create a new Schedule Page that allows you to define customer JavaScript, Jelly and Sever-side script. There are many OOTB example of this and the Wiki has good info on it. Once you have your schedule page you should then define a NEW macro/decoration that opens your new page
