Just try to get your application in as early as possible. The only time you are truly late is when you submit after the deadline (which differs by school, from October 15 to December).
Remember that many applicants take the August MCAT. These scores for this exam are not returned until October. Therefore, the medical schools get their next big batch of applications at that time. It is probably best if your application is in prior to that.
But, the earlier the better in all cases. June is great, July is very good, August is good, September is Ok, October is late, November is very late. December is pointless.