How can I add the public published US holidays, or any other specialized calendar to my iCal?

Details how to subscribe to private or public published calendars can be found on my iCal 101 Tutorial. Click on this link to get to the specific section.

However, there is still the question where do I find the URL for these calendars? If it is a private published calendar you need to asked the owner to provide the link (URL). As to public published calendars, Apple has a download page that contains a large number of special Calendars.

As to the original request for the US holidays URL, it is http://ical.mac.com/ical/US32Holidays.ics. You can either cut and past it into iCal, or go to the specific US Calendar page to download it and open it with iCal.