The next big events on the schedule occur in a few days. For the newbies one is important, for those in the playoffs the both are important.
The first is roster expansion. This allows rosters to expand to the full 40 man roster if desired. Most call up rookies to evaluate how they perform at the ML level. To do so puts them on the 40 man roster if they are not already on it. Those not already on the 40 man roster starts their clock and will use an option at the end of the season. That action may not be desirable at this time and caution may be the word on that. Those with three years experience can be a little trickier. Those with three years experience and probably ML ready, do you wait til next season when you would need to put him on the 40 man roster or give him the experience with the ML club this season? The catch is, if you send him back down at the beginning of next season, the best time btw, then you will have to hold him at the minor league level more games to slow his ML service time to catch another year. Be careful about calling up players that are out of options also. Those players would need to clear waivers to send them back down. One other minor thing with call ups is that you will need to carry the minimum roster requirements at each minor league level which is 21.
A hint here for the newbies, if your minor league teams are playoff bound, especially AAA, that experience actually helps more than a call up IMO. Also, at the end of the season you might want to move a player up to put them on a minor league playoff team.
The second is postseason roster. This is for playoff teams, players must be on the 40 man roster before this time to be eligible for postseason play. Injuries do happen during the playoffs and sometimes a manager may want to change the 25 man roster, however the only time the rosters can be changed is before a series starts, not during.