Vivek, Ya its database specific.. and you are right, by mistake i had given syntax for future saturday...
In SQL, you can use Dateadd or Datepart functions instead Next_day..
Last_Sat = DATEADD(day, -1 - (DATEPART(dw, GETDATE()) + @@DATEFIRST - 1) % 7, GETDATE()) AS LastSaturday
If you want to apply filter for all the blocks, better to create a object at universe level and use it in your query filter -- Web Intelligence.