I think it should be possible to have a 3rd Variable in the Weekday function that lists a bunch of Public Holidays and removes them from the calculation.
That would be a great addition to our current Formula selection! We have been able to account for Public Holidays using Application Scripts, however, a function that does it would be a great enhancement. I have added a feature request in your name, and you will be notified once there is an update on the status of that request.
Let us know if you have further questions!