Going into each month of the regular season, each team is limited to a maximum of 35 players on its roster. The roster is frozen for each month of league play (any roster changes take effect the next month). For each series within a month, up to 26 players may be selected to be active for the series. A different set of 26 can be designated for each series in the month if desired. For the final two months of the season (October and November), all 35 players may be activated for each series.
The list of "cut-down dates" for each month is shown below. Note that team rosters may temporarily exceed 35 players between cut-down dates, especially between November and April (the off-season), but must be reduced to the maximum roster size by midnight on each cut-down date.
| Max Roster Size | Cut-Down Date | For Games in | Rosters Affected By |
| 35 | March 25 | April, April/May | Trades |
| 35 | April 25 | May | Trades, Waiver Claims |
| 35 | May 25 | June | Trades, Waiver Claims |
| 35 | June 25 | July | Trades, Waiver Claims |
| 35 | July 25 | August | Trades, Waiver Claims |
| 35 | August 25 | September | Trades, Waiver Claims |
| 35 | September 25 | October | Waiver Claims |
| 35 | October 25 | November | Waiver Claims |
Note: Late in the season (usually after the final trade deadline for the season), teams typically begin to make trades which will take effect the next year. During this period of time, two sets of rosters will be maintained, one for the current season being played and one for the next season. Any roster moves made which affect a team's roster for the current season obviously also affect that team's roster for the next season.