To add a US Holidays calendar to DAKboard, copy the following URL:
https://www.calendarlabs.com/ical-calendar/ics/76/US_Holidays.ics
There are many other countries' holiday calendars available here: http://icalshare.com. Find the calendar you're looking for then copy the calendar URL.
Back on DAKboard, edit the calendar and paste in the copied URL:
Save your changes, and preview your screen to see the holidays on DAKboard.