Starting on January 23, Activision is planning a free event for Call of Duty: WWII that coincides with the release of the Resistance DLC. This is apparently limited to some in-game event that will provide you new free weapons, the return of the Demolition mode, new Resistance gear to go with the new Resistance Division, double XP, and more.

There is a trailer that should give you a good overview of the content that will be available. In addition, the Resistance page (that I already linked but am linking again here) provides even more detailed information on the DLC, including the content that you'll be able to get over the course of five weeks. The freebies look to change every week, including the modes that you will be able to earn the double XP in, so be sure to check that out!