There is a very easy solution. the N900 calendar can import ics files.
So what you do is:

1. 1. get hold of a public calendar in ics format, e.g. http://newz.at/files/2006/08/feiertage_de20092015.ics for public holidays in Germany
2. 2. download this file with your N900 using the browser
3. 3. fire up graphical file manager, locate the file and click on it
4. 4. then you will be asked into which calendar you want to import the file. I choose a separate calendar 'public holiday' Thats it.
5. 5. Make sure that this calendar is also shown in your calendar (settings -- visible)

Nice!

tredlie