According to Sledgehammer Games, January 23rd-26th will be a double XP time period in multiplayer, and marks the release of 2 new game modes: Hardcore Mosh Pit, and Comeback 24/7. The day of which specifically these are being released has not been said, but it will likely be the 23rd. This starts specifically on Friday at 9 A.M. P.S.T. and ends Monday at 9 A.M. P.S.T.
Meanwhile, still no word on when the AE4 glitch for digital download season pass holders on PS4 will be fixed so the AE4 can actually be used.