The day he/she was born? Or, for a lesser version of the same effect, the day that the character became embroiled in the campaign or scenario, thereby undoing any of the hard work that character has done towards the goal, tempting him/her further into the fae's clutches for a 'solution'.
Or, hold it in reserve until the group is tied up in a situation that the faerie, or one of the faerie's contacts, might have interest in, and have the faerie claim that day back so that the character cannot interfere. You might want to warn the player (not the group) a little before, and tailor the encounters to your now weakened party. If you really want to rub it in, have an encounter that highlights that PC's skills, then remove him/her.
Finally, have the faerie call him in to take care of some business in the Nevernever. Thanks to temporal shenanigans, however long the scenario in the Nevernever takes, the character or group returns to find exactly one real day has passed.