That was my experience - if you seach for secondary info on schools for last year (there should be a whole thread of links somewhere), then you can get an idea of dates. But even though primaries go in early June I didn't see too many schools sending out secondaries until mid-July (very few), late July (lots), and early August (more). (you could also check out some mdapps - some people have their own personal timelines for each school)
And interviews don't really start until late August or early September. So getting everything in a month or so before can still get you early interviews and early acceptances.