Barca had been due to host their bitter rivals on 27 October (AEDT), only for the game to be postponed amid concerns over civil and political unrest in Catalonia.
The Royal Spanish Football Federation (RFEF) accepted a joint request from the clubs to reschedule for December 18.
LaLiga's preference was 7 December, so it could stage its showpiece fixture on a Saturday, and confirmed it would assess the possibility of legal action following RFEF's announcement.
However, a tweet from the organisation on Wednesday confirmed a 19 December Clasico with a kick-off time of 6am (AEDT)
Barcelona and Real Madrid are level on 25 points at the top of LaLiga after 12 matches this season, with the Blaugrana leading on goal difference.
Atletico Madrid are a point further back in third but have played a game more as a result of the Clasico postponement.