I succesfully managed to include holidays from Google with the following procedure:

- Export desired google holiday calendar into ical / ics -file
- Import it to your google calendar or make a new one and import
- Go to: https://www.google.com/calendar/iphoneselect and select the calendars you want to sync
- Create a new CalDav account with your google credentials using URL: google.com
- Save CalDav and manually sync.

I had a problem with the initial sync which resolved itself by rebooting the N9.