As with most issues there are many ways to accomplish this. The Pega Robotics Advanced tab in the toolbox includes a DateTimeUtils component. This component give an automation developer access to all of the functionality of the DateTime object in .net. You could use the DateTimeUtils component to calculate holidays with know formulas (e.g. Memorial day is the last Monday of May). The Script component can also be used for this, as you can write C# code to perform the calculations using the DateTime object in .net.
Other ways to accomplish this.
1. As you suggest, they could be defined in an Excel spreedsheet
2. You could define them in an XML document that is accessible by the solution