More on Sunbird and Subscriptions
Apple hosts several calendars that you can subscribe to with Sunbird or iCal at: http://www.apple.com/ical/library/.
To subscribe to a calendar with Sunbird, simply use
File - Subscribe to Remote Calendar. Enter a name for the calendar, and the url (for US Holidays use: webcal://ical.mac.com/ical/US32Holidays.ics for instance). Don't check automtaically publish changes (because you won't be able to).