To add a holiday calendar, please go to → Calendars & Task lists section and a) add the calendar directly from the calendar list or b) use the '+' button provided in the upper right corner.
Please note that, you will see in your calendar only the strictly official, public holidays.
Hint: If you happen not to find a suitable holidays in our list or look for other special calendars (for example lunar calendar), you will need to use other sources. To add the national/religious holidays or other special calendars to your calendar, it may be the best to you use a Google account. In your Google Web Calendar click on the small grey arrow behind → Other calendars and choose → Browse Interesting calendars in the popup menu. On the following page you are able to browse through and subscribe to special calendars. After the synchronization with your mobile device you should be able to select this calendar for the preview of the Business Calendar!