A position starting in August almost certainly expects you to immediately teach courses for the academic Fall Term. As a junior/first-year instructor, the courses and their general expected learning outcomes will typically be dictated to you, and you will likely be expected to use whatever textbook and resources were indicated while students registered for classes during the Spring and Summer.*
In order to make this expectation reasonable, a few concessions are made. Normally instructors are expected to handle academic advising for a certain number of the students in their subject area, but it's common to waive this requirement for first year instructors. This allows new instructors to devote more office time to preparing lesson plans and the like, and helps avoid advising mistakes from instructors who are not yet familiar with the degree plans in their area. You may also have one course fewer on your load than is normally expected, or you may be granted an extension on when your full syllabi must be posted. Finally, you may be given access to materials from prior or concurrent sections of the same course as taught by other instructors.
But, when it comes down to it, that first year you'll likely still find yourself writing lesson plans the week and day before delivering them. In the current climate, be grateful for a tenure-track position at all :/
*Note that this isn't a matter of infringing your academic freedom; it's just that there won't be time for you to start from scratch, and you should still expect full academic freedom as you get your legs under you in the new position.