I've been looking for something like this but thought I had to find an external VCAL file to import into my Google Calendar. Apparently Google Calendar has a whole bunch of useful public calendars. For example, UK Holidays.
The rest is easy and needs no explanation. I did however spot one "bug" or perhaps I just confused myself. When I went in the second time to take a screenshot of the list of public calendars available it had mistakenly unselected the ones I selected a few seconds ago. Perhaps a caching problem.