- On iCal's menu bar, choose Calendar / Find Subscriptions...
- You will be taken to a page on Apple's website that lists shared calendars.
- For US Holidays, click here. For other countries you will have to browse around some in the Alphabetical tab.
- When you have found the calendar you want to add to iCal, click the Download button.
- You will be taken back to iCal. Click Subscribe.
- If you have MobileMe and share your calendar with iDevices and/or other Macs, change Location from On My Mac to Calendars. (If this doesn't work, make sure that Calendar sharing is checked on the MobileMe preference pane.)
- Change Auto-refresh to Every week. This will make sure that you have holidays on your calendar for the years ahead.
In addition to holiday calendars, you can also subscribe to US sports team schedules, US movie releases, moon phases and several other types of calendar.
Also check out the iCalShare.com website that claims over 3500 shared calendars in 40 categories