Applying to professional school is a long process, so it is in your best interest to be informed and prepared as early as possible.
In general, applications are available in early May and should be submitted between June and September of your junior year. All the preprofessional course requirements should be completed at the time of application.
An early application can enhance your chances for admission because it assists the admission committee's activities, demonstrates your enthusiasm and preparation, and allows for interview scheduling before the end of the year.
Steps to prepare include collecting and placing letters of evaluation into confidential file, taking the admissions test (MCAT, DAT, OAT, GRE, PCAT) in the spring of your junior year, and submitting the initial application no later than late August or early September (by the beginning of the senior year). Make your application file as strong as possible. As you are well aware, admission into professional programs is highly selective.
Know the deadlines. Different schools have different deadlines. Admission committees meet through the fall.