Application can be submitted on AMCAS starting June first but opens for editing in May. So you have plenty of time to get started early, even if it’s just filling out grades and activity hours.
SDN Consensus: If you want to be early, you will want to submit your primary before July. Most “on time” apps will probably submit primary between July and August. Submitting your primary from August to September is starting to push it but not a death sentence. Submitting in September is definitely late.
Check out the link in my sig to see when most interviews go out for the MD side. MD/PhD programs tend to start interviewing a little later than MD but also keep in mind that there are also fewer interview dates.