When a schedule has been created in Zendesk without any holidays, there are some flow-on effects which can occur:

• Service Level Agreement's (SLA's) will not take into account public holidays and will continue to function on tickets
• Workflows (business rules) which utilise the schedule or business hours will not take into account public holidays and will continue to operate

Solution:

You can easily add holidays to your schedule from the Schedule settings under Admin Center within Zendesk Support.

• Navigate to Admin > Schedule > Select the Schedule to edit > Click on the Holidays tab > Click Add holidays. 
  • Refer to this Zendesk Support guide for more information: Setting your schedule with business hours & holidays

Please note that to date, Zendesk hasn't provided an easy import tool for uploading holidays, so this unfortunately needs to be manually entered.