Hi there @MattonKNIME,
and welcome to KNIME Community!
Seems there are two things you need to do:
- determine the first business day of following month based on today’s date (i.e. execution time)
- filter your input file according to that
My approach to this would be to keep things as simple as possible. That means that you take 10-15 minutes of your time, take calendar and write down first business days for next 2 years in a txt file. This makes sense as it does not require hours of work, you are not doing this for 100 countries (rather only one) and it simplifies workflow a lot. All you need is to check that this file is up to date if calendar changes in terms of holidays 
Then you take this file as an input, filter it so it includes first business days of following month and create a flow variable from it. Part two is then simply filtering your input file depending on that flow variable using Rule-base Row Filter node.
Here is example workflow:
Input files are inside data folder within workflow directory and workflow relative paths are used so you can test it.
If any questions feel free to ask.
Happy KNIMEing!
Br,
Ivan