Calculating End of Week Date

Try this

var date RawDate = If([Category] = "PNP", WeekdayAdd([Equip Reqd By],-21), 
WeekdayAdd([Equip Reqd By],-14));

LastDayOfPeriod($RawDate, Weeks(1),Date(2017,5,20))

What does is to take the Period starting on a Saturday (it does not matter which Saturday) and based on a period length of 1 week, it calculates the Last Day of the Period, which of course must be a Friday if the 1 week period starts on a Saturday.

This is a great solution.  Works.  I noticed  that you have used certain date (2017,5.20).  Will that need to be amended at any stage? 

That field will only needs to be changed if the world goes to a 6 or 8 day week as the new Global standard.  That seems unlikely any time soon!  So no, it can be any Saturday even one 200 years ago or in the future.

Haha  cheers