The Speedsters event is going on from 8/22 - 9/3 starting at 3pm est. All players need to have a speed rating of 75 or above; they can be bronze, silver, gold, or diamond. All other players have to be bronze or common; like the pitchers, catchers, and 1B most likely).
If you miss this event, keep checking for a later event, I think there is an event every month, but I'm not sure on that.
Hope this helps!