When is the day of Septmeber call-ups?

Question

Can teams really carry a 40 man roster?

And can they take the call-ups into the playoffs?

Answer 1

From September 1 until the end of the regular season, teams use expanded rosters, so they are allowed to have anybody from the 40-man roster play in games during that final month.  

Answer 2

Call-ups begin on Sept 1.

Any one from the 40 man roster can play in a major league game.

The call-ups can be in the playoffs because they are part of the 40 man roster but a team is limited to 25 players on the roster for the playoffs.  The roster can change each round of the playoffs.

Answer 3

Additional players can be called up beginning 01-September. Some teams bring up kids right away, others wait for minor league seasons (including postseasons) to finish.

In most cases, call-ups cannot participate in the postseason. There are options for replacing injured players, however; the best known of these in recent years is Francisco "K-Rod" Rodriguez, who made his major league debut on 18-Sept-2002, and was a key piece of the Angels championship team that year.

Another, even more unusual case was Mark Kiger of the 2006 Oakland Athletics, who made his major league debut as a defensive replacement in the Division Series. (To date, he has not yet played in the regular season. He's currently in the Seattle organization.)