When you add in new holidays to your Calendar it will not affect already existing Incidents.
If an Incident is set to breach on the 1st of July and you add in a holiday for the 1st of July, the Incident will still breach on the 1st of July. If you however create a new Incident with the same Response Level this will count the 1st of July as a holiday.
I would therefor recommend that you add in your holidays enough time in advance as your longest SLA is.
So if your longest SLA is set to breach after 100 hours you will need to add in your holidays at least 100 hours before the day you wish to add in as a holiday.
Should you need to add in a holiday with a short period of time before the holiday takes place then there is a way to have your Incidents recalculate the holidays.
This is done by using a "Stop" and "Start" Action on each Incident. (if going to "With Customer" and back has a a stop and start Action in your Process, then you could use this)
This will force your Incident to look at the Expire time again and therefore recalculate taking into concideration the new added holiday.
The Start and Stop Action on the Incidents needs to be done induvidually per Incidents and cannot be done via a bulk action.
See also this article to check that your shifts are entered correctly: Creating Holidays within Service Level Management