Barca had been due to host their bitter rivals on October 26, only for the game to be postponed amid concerns over civil and political unrest in Catalonia.
LaLiga's preference was December 7, 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 organization on Wednesday confirmed a December 18 Clasico with a kick-off time of 2pm ET / 11am PT.
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.