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As a Supervisor or Administrator, you should have access to iceAdministrator where you can update the Holiday list for the workflow that calls and contacts follow. 

Below are the steps pulled from the user manual which outlines what you need to do in iceAdministrator. 
 
Adding a Holiday
 
Holidays are used in the Check Time Schedule action. If a contact passes through this action during a day that is defined as a holiday, the contact is routed to the 'holiday' workflow (e.g., a workflow that plays a holiday message).
 
1. Right-click on the Holidays folder under the Workflow folder. A menu appears.
 
 
2. Select New Holiday from the menu. The right side of the iceAdministrator window displays the properties for the new holiday.
 
3. Use the dropdown calendar on the Properties page to select the holiday. The default value in the calendar is the current date.
 
 
4. From the file menu, choose Save. To cancel the save operation, click Cancel on the Progress dialog box.