In football, there is a term referred to as transfer windows.
A transfer window is a specific time during a football season when clubs can buy and sell players. The transfer window was introduced in 2002 as a compromise between FIFA and the European Commission to stabilize club squads and preserve contractual stability for players and clubs.
Most European leagues have two transfer windows, one from the end of the season to August 31st, and then another in January.
To complete a transfer, a player is registered with the new club through FIFA during the period of the transfer window.
If transfers are done outside the transfer window, new signings must wait until the next registration period to play for the club.
This was the reason why Sam's new club registration was scheduled to be completed in January when the transfer window finally opens.