I've read this question before, but didn't see it as a feature request.
We've installed several PRTG instances at customers' sites and they only have SLAs to be maintained on weekdays. Now PRTG doesn't know any holidays and so it's the warnings on these days that come out.
At the moment we always go there and adapt the Sheduler and take out the days in the week that are holidays. But this is a high effort, which can be solved easier, if necessary, by excluding days in PRTG at the beginning of the year or by finding the days in PRTG itself.
Please help to find a Solution here