So, I got one of these in the mail:
You can click on it to enlarge.
It's a post card you'll receive a couple months before your driver's license expires in Japan, which is coincidentally aligned with your birthday. So if you thought it might be a congratulatory message from your prefectural traffic safety association, I'm sorry to break the bad news.
The middle section explains when your renewal period is: It encompasses one month before and after your birthday, and if the last day falls on a Saturday, Sunday, or holiday, you can also go the next weekday, though I wouldn't recommend putting it off that long.
It tells you what kind of license you stand to receive: In my case, a blue "normal" license (my last one was a green "beginner" license since it was my first). And it also tells you what kind of course you'll need to take when you renew (somewhere between 30 minutes and 120 minutes depending on your driving history and the number of times you've renewed) and how much the renewal process will cost, which in my case was 4,000 yen.
It also includes a short list of what you need to bring when you renew.
The right-hand side explains which locations you can renew at, and what times renewals are available. This will vary by prefecture and by licensing center. For me, renewals were held Sunday through Friday, with three available class times each day. You must arrive promptly during one of the check-in times (受付時間) in order to have enough time to complete the application process before class begins. If you arrive too late or your application takes too long, you'll have to wait around a few hours for the next class time or come back another day.
On the back of this postcard are details about stuff like public transportation options for accessing the licensing center, what size of photograph to bring if you want or need to bring your own photo, details about the refresher course you'll probably have to take, and instructions for renewing your license from out of prefecture if you're currently living somewhere else (but have reason to maintain a license issued by the notifying prefecture, as might be the case with a student or a "business bachelor").