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.
Thank you for reaching out! I have just checked our system, and it looks like the feature request to allow removing public holidays from calculations through a formula enhancement is currently logged with our Product team. To make sure you are notified once it is released, and to add another request to the functionality as well, I went ahead and logged a feature request in your Organization's name and added your email address to the request on the functionality of being able to remove public holidays from calculations through a formula enhancement with our Product team, for them to consider implementing in a future Project.
Let me know if I can help with anything else,
Taras Gonchar commented
Do you have an addition to the formula already working?
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!