Worldwide holiday calendars take a regular calendar and mark it up so you can see that tomorrow, for instance, is Bolivia Independence Day. I added two worldwide holiday calenders, one by Country Reports (all countries on one calendar) and one Time and Date (a different calendar for each country) to the Calendars entry.
Thanks to Margaret Gross and Kevin Adams for recommending the Country Reports and Time and Date calendars in a post on the BISLIB listserv. And yes, they do have Christmas in Croatia, two days of it apparently, celebrated on December 25th and 26th. Who knew?