I'm trying to represent days of the week as a string

I'm trying to represent days of the week as a string. However in the example above if {rental.mon_qty}=1 it puts MO but it does don't put all of the other abbreviations that have a 1.

Here is my formula:

if{rental.sun_qty}=1 then "SU " else

if{rental.mon_qty}=1 then "MO " else

if{rental.tue_qty}=1 then "TU " else

if{rental.wed_qty}=1 then "WE " else

if{rental.thu_qty}=1 then "TH " else

if{rental.fri_qty}=1 then "FR " else

if{rental.sat_qty}=1 then "SA "