Came across this on FlyerTalk
There are no published T&Cs so best just to have a play around, I have got it working for flights in Sept/Oct in business class.
I have confirmed it is working in The Residence which is a saving of $25,818 from SYD-LHR return.
More info here
NOTE: Don't forget you can earn 4x Etihad Guest Miles on Business and First fares and 3x on Economy fares (via registration).
Can confirm both codes work on flights to London next month. Why have you tempted me with this? Whyyyy? On the searches I made Tuesdays seem cheaper than Wednesday for some reason.