I think it's still possible to program the calendar app in macOS (might need some permissions) with AppleScript. At the end of each year, I run a Python script

I think it's still possible to program the calendar app in macOS (might need some permissions) with AppleScript. At the end of each year, I run a Python script which through AppleScript creates an event at the beginning of each month that reminds me to send some papers to my accountant. Instead of just setting the event on the 1st of each month, the script chooses the first working day of the month, taking into account weekends and bank holidays. Considering you can create multiple calendars, the script (with a cron job) could read the events in the "Exceptional" calendar and update the "Work" calendar. Something similar might be possible with Google Calendar through the API.