Clubs normally have a 26-man active Major League roster but are allowed to recall one additional player who is on the 40-man roster to serve as the 27th member for the day of a doubleheader. That player accrues one day of MLB service time and is returned to the Minor Leagues following the completion of the doubleheader. This rule is in effect from Opening Day through Aug. 31. From Sept. 1 through the end of the regular season, all clubs must carry 28 players. The active roster reverts back to 26 players during the postseason.
Players who are optioned to the Minor Leagues typically must remain there for a minimum of 15 days before they are eligible to be recalled to the Major League roster, but if a player is serving as the 27th man for a doubleheader or replacing a player who has been placed on the injured list, he can be recalled without having spent a minimum of 15 days in the Minors after being optioned. Teams also are permitted to call up a 27th man on days when they are completing a previously suspended game and have another game scheduled after that, but the 27th man is only permitted to play in the regularly scheduled game that day, not the contest that was previously suspended.
Prior to 2020, the active roster size from Opening Day through Aug. 31 (and during the postseason) was 25 players. A team was permitted to carry 26 players for doubleheaders and on days when they had to complete a previously suspended game.
On Sept. 1, teams were permitted to carry up to 40 players on their active roster until the start of postseason play, at which point the maximum reverted back to 25.