How rare is it to be able to get it for the return as well with a few days to a week in between ?
Won't be free again on the return, so if you are ready to shell out some money I am sure it is likely on more popular routes.
I have to book on the 14th, or the flight has to be on the 14th?
Flight has to be on the 14th.
Book it. I believe They only have flights in the next 24 hours
-1. They have flights up to 48 hours in advance, and you can book it on the 13th for travel on the 14th.