Like an eclipse or an obscure comet sighting, tomorrow is the only day on the calendar you may pick up this Silver PSN trophy or 40 Gamerscore Xbox Live achievement, in MLB 2K10<. So be ready when it comes.
The trophy/achievement is "Mid-Summer Classic", and all you have to do to collect it is play the All-Star Game in MLB 2K10's MLB Today mode, which allows you to play the scheduled games of the day with their real world lineups.
If you miss it, I guess you can try again next year, but if you have the chance, why wait?
Achievement Reminder: MLB 2K10's Mid-Summer Classic [Xbox360Achievements.org]