http://www.earthcalendar.net//index.php  A calendar of all holidays around the globe.  Searchable by date, country, religion, etc.  Includes things like Receptionist Day and Towel Day.

http://www.lightdev.com/holcal/ current month's holidays

http://shagtown.com/days/usa.htmla good list of US holidays with descriptions (also has other countries' holidays if you go to the home page)

http://www.sunniebunniezz.com/holiday/ a list of US holidays with links to ways to celebrate if you dig into the site

http://www.me2u.com/LoveLore/Calendar/ a fascinating calendar listing daily excuses to celebrate (including things like Postcard Week, Limmerick Day, and Visit Your Relatives Day)

http://www.mynameday.com/index.html A site where you can find your name day (mine is March 31 and Colin's is June 9th this year, btw)

http://en.wikipedia.org/wiki/Category:Holidays_in_the_United_States the trusty wikipedia

http://familycrafts.about.com/cs/holidays/l/blspecdays.htm ways to celebrate holidays–I think this one is my favorite (includes National Child Care Providers Day and Twilight Zone Day on our anniversary and it's American Bike Month, Asparagus Month, Egg Month, and National Hamburger Month…who knew?)