Here is a simple way to add holidays to your iCal.
- Open this link in your web browser – webcal://ical.mac.com/ical/US32Holidays.ics
- Select iCal and hit OK
- A box appears – Subscribe to Calendar, click on Subscribe
- A new calendar will be created – US Holidays and hit OK
Holidays should now appear in your calendar. Just make sure you have a check mark next to US Holidays.