Now. You should start sending reminders today or tomorrow that you need it by the time ERAS opens (to be safe, you can request by September 10th). Almost all of us who have been through this have had to remind our letter writers as they are often busy clinicians/researchers, it's just part of the process.
I am in the same situation. Do you have any tips on how to go about asking in the most respectful manner possible? Would simply sending emails be too impersonal?
I already asked for this letter (in person) 2-3 months ago while I was still working with this attending. I got a warm response at the time and have been told that he tends to need reminders. I have emailed 2 reminders since then, but haven't gotten any reply. I have his cell number, but that seems like too much.