This standalone download contains the most recent holiday definitions for Outlook.
The download title specifically mentions Outlook 2007 and Outlook 2010 but you’ll find it also works with Outlook 2003 and Outlook 2013.
This definition file is available in various languages and contains holidays for 88 countries up to the year 2028. It holds corrections over the one that is currently included with an up-to-date Office 2007 and Office 2010 installation.
This .hol file still contains incorrect dates for Whit Sunday and Whit Monday for 2015. This applies to the dates defined for English regions and Germany (Pfingstsonntag and Pfingstmontag). To correct these see Microsoft Support KB2764690.
For details about how to use this hol-file see: Holiday updates for the Outlook Calendar.