For MLB postseason roster eligibility, the key roster cutoff is August 31. A player generally must be in the organization’s qualifying roster pool by the end of that date to be eligible for postseason play with the club.
The rule is tied to the 40-man reserve list and certain injured, bereavement, or restricted-list statuses. It prevents teams from adding outside players late in September and gaining an unrestricted playoff reinforcement after the regular season’s roster picture is largely set. The practical deadline is often described as before September 1, with the exact cutoff time specified in MLB rules.
This is not the same as the regular-season trade deadline, which occurs in late July, nor is it the date the postseason begins. A player acquired after the cutoff may still help during the regular season but normally cannot appear for that team in the playoffs. Exceptions can apply, especially when an eligible injured player is replaced with approval from the Commissioner’s Office. The rule has produced notable cases of late-season acquisitions being unable to participate in October.