Upvote:2
Your Korail Pass is a ticket already valid for travel on any Korail train in Korea (although a seat is not guaranteed!), so I presume you are asking about reservations.
There are less than 30 days to go until September 10, so you can already make reservations online. You can also make them at train stations, both in advance and on the day of travel.
In my experience, outside peak holiday periods, there is no need to make reservations in advance, just arrive at the train station a bit early -- or for frequent services like the Seoul-Busan KTX, you can just get the next train.