Calculate weekday based on due date

KnowNow ,

The simplest way I would do this is to use the SWITCH function:

ClosestTueorThurs = SWITCH(
                        TRUE(),
                        WEEKDAY( [InvoiceDueDate] ) = 1, [InvoiceDueDate] + 2,
                        WEEKDAY( [InvoiceDueDate] ) = 2, [InvoiceDueDate] + 1,
                        WEEKDAY( [InvoiceDueDate] ) = 3, [InvoiceDueDate] + 0,
                        WEEKDAY( [InvoiceDueDate] ) = 4, [InvoiceDueDate] + 1,
                        WEEKDAY( [InvoiceDueDate] ) = 5, [InvoiceDueDate] + 0,
                        WEEKDAY( [InvoiceDueDate] ) = 6, [InvoiceDueDate] + 4,
                        WEEKDAY( [InvoiceDueDate] ) = 7, [InvoiceDueDate] + 3 )

I have made the assumption that if your InvoiceDueDate falls on a Tuesday or Thursday, the closest day is that date.  You can alter the Switch Statement accordingly if that assumption is incorrect.
I have used the default type to indicate Sunday = 1.

Hope this helps.

Regards,