Google Calendar adds holidays, sports calendars, birthdays
And if you click the "More" tab you'll see an option to add the birthdays of contacts in your Google address book.
The beauty of Google Calendar is that each of these items is added as a separate calendar. Want to see your personal appointments without all the clutter? Just unselect the sports, holiday, and birthday calendars in your sidebar temporarily to make those appointments go away.
In related news, if you're too lazy to add all the appropriate holiday calendars to your site and just want to figure out whether it's a holiday today or not, there's a single-purpose web site for that. It's called... are you ready for this? IsItAHolidayToday.com. It doesn't have the most comprehensive list of holidays I've ever seen, but if it did, you'd probably see a holiday every single day.