Lauren:
At first, I too was a bit disappointed with the holiday and event options in iCal. The help wasn't too 'helpful', in layman's terms anyway. But then I turned to the great community Mac people.
Turn's out that iCal so elegantly flexible & powerful that sometimes we are looking for something to be harder that it is. The subscription feature is the key. With the subscription you can link to other calendars and have them auto update. These can be from other people or your own calendars. It looks like ical.mac.com ... potentially has a calendars to subscribe to. But that requires a .Mac account...
But in the age of the internet there should be lots of other options. So I googled and discovered Google Calendar and their supportive community. They support the iCal, and other, formats. (See link at end) Here you can search for a wide variety of calendars. I found a US holiday and Christian calendar. I'm confident that other sites are out there.
Here's what I did. After finding a calendar you like, you can either:
A) Subscribe by control-click on the iCal link, "copy link" and paste into the iCal subscribe dialog. This will get you the Auto updates but no changes allowed.
B) Download and import by control-click and "save the linked file" to your local computer. Then just drag the iCal file onto your iCal window and you'll be prompted with options for importing the calendar.
This is all so very cool. We really can't blame Apple too much, think of all the complexities and varieties of calendars around the world, it's better that they've enabled an open format and easy to use tools for thinking people to use.
Disclaimer: I don't work for Apple today, I used to some 20 years ago as in intern. Today, for work, I'm chained to a Windows OS due to a narrow-mined IS department. However I not spending my own money with Bill and friends.
Good luck with your calendars.
Goodle Calendar: Holiday page:
http://www.google.com/calendar/render#