Transfer window is the term given to the period in football when a football club can transfer players either in or out of their playing staff. The window was introduced in response to negotiations with the European Commission. The system has been used in many European leagues before being brought into compulsory effect by FIFA during the 2002-03 season. There are two windows per season, 1 July until midnight of 31 August and one from midnight on 31 December until midnight on 31 January the following year.

Transfer deadline day

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The day upon which a window closes is known as deadline day. Deadline day is usually one of the busiest days of the transfer window. Typically this day generates a rash of media interest and can also see big moves; for example, on 31 August 2005, Michael Owen moved from Real Madrid to Newcastle United and on 31 August 2004, Everton's Wayne Rooney signed for Manchester United. If the Last day of a transfer window is on a Saturday then the deadline is usually extended to the Monday for business reasons.

Reference

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Transfer Windows Explained (Official Website of the FA Premier League)

Football (soccer) terminology