Saturday is September 1, which means all 30 MLB teams are now free to expand their active rosters from 25 players all the way to 40 if they choose. Most teams do not play with a full 40-man active roster. Most carry 30-35 players, and there is no rule against teams having different roster sizes for a game or a series. If one team has 28 players and another has 37, so be it.

September call-ups have been around forever, and they are somewhat controversial. Many fans and analysts don't like that the last month of the season -- the most important games of the year -- are played with extra players. It seems inevitable that a rule change will be made at some point, perhaps limiting the number of active players per game in September. I think September call-ups are great, personally.

Teams use September call-ups in different ways. Rebuilding teams will often bring up young prospects and essentially give them a month-long audition in the big leagues. Contending teams tend to stick with their regular players, at least until they clinch a postseason spot, so most of the time they use their September call-ups in blowouts or emergencies only. As for postseason eligibility, there are two things to know about September call-ups:

  1. The player had to be in the organization by 11:59 p.m. ET on August 31 to be eligible for the postseason roster. No exceptions. They didn't have to be on the MLB roster or even the 40-man roster, but they had to be in the organization. That's why there are always last-minute trades on August 31.
  2. Players not on the 40-man roster prior to September 1 can be added to the postseason roster through a substitution rule. He can be added to the 40-man in September and carried on the postseason roster as an injury replacement for a player who has spent at least 60 days on the 60-day DL.

This year most of the attention seems to be on players who are not getting called up this month, specifically Blue Jays third baseman Vladimir Guerrero Jr. and White Sox outfielder Eloy Jimenez. They are arguably the two best prospects in the minors and both are crushing Triple-A, but neither will be called up this month. The Blue Jays and White Sox are manipulating their service time to gain an extra year of control before free agency. That's the only reason they won't be called up.

As for the players who have already been called up as part of expanded rosters -- it should be noted teams call players up gradually throughout September as minor-league teams get knocked out of the postseason -- here are five of this year's most notable September 1 call-ups, listed alphabetically.

Pittsburgh Pirates: RHP Nick Burdi

The Pirates selected Nick Burdi from the Twins in the Rule 5 Draft last winter even though he was rehabbing from Tommy John surgery. Pittsburgh figured the righty's triple-digit heater and swing-and-miss slider were worth a roll of the dice. Burdi struck out 16 in 11 rehab innings and will now get to strut his stuff in the big-league bullpen in September. As with most players on the way back from Tommy John surgery, his control doesn't figure to be all the way back yet, but the raw stuff remains impressive. The Pirates took a gamble last offseason, and now it's time to find out whether that gamble will pay off.

Houston Astros: RHP Josh James

Few prospects in the minors have broken out this season as much as Astros righty Josh James. His fastball jumped from the low-90s into the 95-97 mph range, and he even touched 100 mph a few times. A quality slider and changeup have helped James strike out 171 batters -- third most in the minors -- in only 114 1/3 innings split between Double-A and Triple-A. James posted a 3.23 ERA in those 114 1/3 innings and will make his MLB debut as a starter Saturday in place of the injured Lance McCullers Jr.

St. Louis Cardinals: C Carson Kelly

The Cardinals have a problem on their hand. A "problem," I should say, because too many good players is not really a problem. Yadier Molina is still going strong -- and signed for another two seasons at $20 million a pop -- and they have an MLB-ready heir apparent in Carson Kelly. Kelly, 24, has been up and down with St. Louis the last two years, and this season he owns a .269/.378/.395 batting line with as many walks as strikeouts (48) in Triple-A. St. Louis is in the postseason race and they figure to run Molina out there as much as possible in September. Kelly is a budding All-Star behind the plate though.

New York Yankees: LHP Stephen Tarpley

The Yankees acquired southpaw Stephen Tarpley from the Pirates as a player to be named later in the Ivan Nova trade two years ago, and after shifting to the bullpen full-time, he broke out this year at Double-A and Triple-A. Tarpley threw 69 2/3 minor-league innings with a 1.94 ERA and 71 strikeouts, and his 68.1 percent ground ball rate is the highest among the 1,011 pitchers with at least 65 innings in the minors this year. He's also held left-handed batters to a .148/.219/.193 batting line, so Tarpley gives the Yankees a potential left-on-left matchup guy and ground ball specialist down the stretch.

Atlanta Braves: RHP Kyle Wright

Kyle Wright, the No. 5 pick in the 2017 draft, will be the latest high-end pitching prospect to debut for the Braves this season. They've unveiled a ton of them. Wright flew through the minors this summer, striking out 133 batters with a 3.46 ERA in 138 innings between Double-A and Triple-A, and Atlanta is planning to use him out of the bullpen. As a starter, he works at 95-97 mph with two wipeout breaking balls in his slider and curveball. In short one-inning bursts, Wright could be a difference-maker for the Braves in September a la Francisco Rodriguez and the 2002 Angels.