from premier league -
What was life like for Premier League clubs before the transfer window?
Players could be traded throughout the season up until March 31. Beyond that it was felt that allowing transfers could undermine the integrity of competitions.
For example, a team going for the title might suddenly acquire players from a club with nothing to play for at the end of the season on short-term contracts, not something that was really in the spirit of a season-long league competition. But the rest of the season you could lose, or gain, a player at point.