Can I retroactively add holidays to a schedule?
It is not possible to add a past holiday to one of your schedules from the Zendesk Support agent interface. The system will not let you pick a past date, the first date you can select is tomorrow:
The way that we handle ticket metrics in Zendesk is by updating information in the moment based on the current settings. We're unable to recalculate metrics that have already occurred in the past. You will not be able to modify handled times for past events.