How do I add holidays?
1) Holidays can be added to iOS as a subscribed calendar and it will show in Agenda. The below assumes you're looking for U.S. Holidays...
Settings.app->Mail,Contacts,Calendars
Add Account
Other
Add Subscribed Calendar
Enter the following and tap "Next"
ical.mac.com/ical/US32Holidays.ics
2) Alternatively, if you're using Google Calendar, add the holiday to that calendar, by following these instructions followed by Google.
http://www.google.com/support/calendar/bin/answer.py?answer=37098