Ana, this feature can be controlled through workshift modification and keep them updated in regular interval. Follow this :
1. Navigate to Administration page
2. ServiceDesk --> Application Data -->Code --> Workshifts
3. Open the existing workshift take it in edit mode.
4. Under schedule add the holiday date in server time format (what you have defined in web.cfg file)
save the workshift and it will exclude those holidays for service type calculation.
Thanks
ArunavaS