From my perspective, you would need to:
1) Have two different media pools with the different retention settings ("Weekly", "Monthly").
2) Have two different tape jobs. ("All Saturdays of the month, apart from the last one", "The last Saturday of the month")
3) Point the tape jobs to the media pools, accordingly.
4) Run PS script executing "weekly" job on all Saturdays of the month, apart from the last one and "monthly" job on the last Saturday of the month.