"League Dates" are the dates within a league's own season.
During the regular season, leagues run at a rate of 10 league days to every one day of real time.
So a league may have a date of June 1st in the morning, and June 11th by the end of the real day.
Before opening day, league days run at a rate of one day in the morning, and one day in the evening.
Throughout post-season, league days run at a rate of three days in the morning, and three days in the evening.
There are certain league dates that are important for managing your team.