HI R P,
As mentioned in 'Schedule a Process chain on specific weekdays' document to use decision step. You can go with it just before setting actual data flow PC steps. Here a help for you.
Logic for decision step-
Create a new custom function/method (say ABC) to calculate last Saturday of every month by using Standard Method 'FIRST_DAY_IN PERIOD_GET' (this gives last Sunday of every month) and use code logic accordingly to get last Saturday.
Now, pick this ABC function from 'User- Defined function' section for your decision step formula.
Hope this help.
Thanks!
Pratish