First go to admin->calendar->holiday management and download the AT
holidays, save it as /path/to/phpgroupware/calendar/setup/holidays.AT.
Also dump the data from phpgw_cal_holidays using mysqdump or phpmyadmin.
Then remove all the AT holidays from phpgw_cal_holidays
In admin->calendar->site configuration there is an option load holidays
dynamically - set this to on, and set load from to localhost.
Go back into claendar, check that the calendar has the correct entries.
If they are ok then upload the holidays.AT file at
http://savannah.gnu.org/patch/?group=phpgroupware