I've just checked and it's 12 noon on the Friday before.
http://www.thefa.com/~/media/files/pdf/fa-competitions-1415/the-fa-cup-rules-2014-2015.ashx?la=en
Section 15 (i) (i)
A “registered player” is one who is either, in the case of a player under written contract, registered with The Association or, in the case of a player without a written contract, registered with a League in which his Club competes in the current season. Any such registration must have been received and accepted by The Association or League by 12 noon on the Friday previous to the date fixed for playing the Round, and the registration must be continuous through to the date of the match.