How do I define that there should be a differential the third and last saterday?

This is what I have now, but it should be the thirth and fifth saterday : 
Level=Differential 2nd-5th sat at 0:05

Is this a good idea : 


        Run = Level=Full 1st sat at 0:05
        Run = Level=Differential 2nd-5th sat at 0:05
        Run = Level=Incremental sun-fri at 0:05



Thanks,
D

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