The OX calendar does not contain any pre-installed holiday data.
If you would like to have public holidays in your calendar, you can use one of several web sites who offer exports of calendar data in iCal format, and import these into the OX calendar.
(For generic information on the iCal format, see VCard and ICal support.)
As an example, we use one of the holiday calendar files provided by the Mozilla Project. For other sources, search for "holiday calendar ics" in your preferred search engine.
To import these holidays:
iCal is a text format, which is readable by humans (even if it does not look very polished). In the next step -- importing -- you will not be shown a preview, so you might want to review the .ics file itself to make sure it contains the data you want.
Please note the current restrictions of this functionality: