Adding holiday to SLA after holiday has passed?

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Hello,

We had an issue where Presidents day was added to our response requirements but NOT to our custom SLA. As a result, incidents from the weekend and Presidents day have been marked as exceeding the SLA (24 hour response requirement, M-F). 

I've been searching for a while but I'm not able to find how to credit an SLA when it is NOT in an active date period. I've tried creating business rules but they don't seem to be triggering.

Any help is greatly appreciated. My knowledge with OCS and rules is very limited but I'm trying to teach myself as I go along.