The application cycle for AMCAS med schools (excepts most schools in Texas, DO med schools, N.Dakota, and a school in Missouri) starts in the first week of June when you are able to submit the application. It ends when all schools are done interviewing and accepting applicants up to the first day of class 12-16 months later.
It is good to apply early in the cycle, as interview spots fill up fast. If you apply in September or October before AMCAS closes to new submissions, you are statistically less likely to get an acceptance.
The application cycles vary for dental, pharmacy, OD, podiatry, etc and you can find out about them in Forums further down on the SDN list.
you want to apply early b/c interview slots will be filled up pretty quickly. it starts in June and applicants who've submitted their applications in a timely matter (by July) can get interviews as soon as September and October. In contrast, if you submit late and your resume isn't that impressive, you could get an interview in February or March or no interviews at all! Around Winter quarter, many schools have already found enough applicants to fill their spots and they're just interviewing for the waitlist...