I'm a Kubuntu user moving to Unity, one nice feature that the KDE panel calendar (the one that pops up when you click on the clock) had is that it indicated my local public holidays. I think each holiday had a red square around it and when you clicked on it it showed the name of the holiday. Can this be set up with for the Unity panel calendar?
The only possible solution I can think of is the following: Evolution, as I am aware, features synchronization with Google Calendar (I use Thunderbird myself so I cannot guarantee how well it works). Google Calendar has plenty of options, including subscribing to public calendars, amongst those I am sure there is one for your region's holidays. The solution would be subscribing to that specific calendar in GC, then synching the calendar with Evolution. All of Evolution's marked dates should automatically be added to the panel calendar entry.
Here are a few links I hope you find helpful:
http://www.google.com/support/calendar/bin/answer.py?answer=37098 - How to add a public calendar to GC
http://ubuntuforums.org/showthread.php?t=1599583 - How to sync GC and Evolution
http://email.about.com/od/googlecalendartips/qt/Move_All_Events_from_One_Google_Calendar_Calendar_to_Another.htm - In case the synchronization doesn't pull the public calendar entries, it needs some further fiddling, but it should work.