On August 31 at midnight ET, three different milestones in the player movement calendar occur. The waiver trade deadline period comes to an end while the postseason eligibility lists are established. These two events mean that the time to load up with players that will be active for the playoffs has passed. From this point on, as far as your playoff roster, you’ve got to go with what you’ve got.
Immediately following that, the dawn of September 1st means that active rosters increase from 25 to 40 players. September callups are on and anyone that joins the club can play during the remainder of the regular season but, should their club make the playoffs, they wouldn’t be eligible.