MLB clubs 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.
Upon being optioned to the Minor Leagues, a position player typically must remain there for a minimum of 10 days before he is eligible to be recalled to the Major League roster. For pitchers and two-way players, the minimum is 15 days. But if a player is serving as the 27th man for a doubleheader, there is no minimum number of days for which the optioned player must remain in the Minors.
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.