In a typical season, 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, 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.
Rules for 2020 season
As part of MLB's COVID-19 health and safety protocols during the 2020 season, teams submitted 60-man pools of players eligible to play in 2020. Each club had a 30-man active roster for the first two weeks of the campaign and 28 for the remainder of the season and postseason. Each team was also permitted a three-player taxi squad for every road trip, giving them immediate options to replace an injured or COVID-19 infected player. Clubs were still able to promote an extra player to the active roster for doubleheaders.
Instead of July 31, the Trade Deadline was Aug. 31. Players needed to be on a club's roster by Sept. 15 in order to be eligible for postseason play.
Rather than a 10-day injured list for position players and 15-day injured list for pitchers, there was a 10-day injured list for all players in the shortened season, as well as a separate COVID-19 injured list that had no minimum duration. The 60-day injured list was reduced to 45 days.