M2H and Blackmill Games are currently running their annual Christmas Truce event for their multiplayer WWI FPS, Verdun.
Inspired by the real events that led to a temporary ceasefire in 1914, the Christmas Truce has been an annual event in Verdun for seven years now. Players can join a special server where they engage in snowball fights, makeshift football matches, and can send special in-game greetings to other players to celebrate the holidays. Everyone who participates also gets a commemorative medal in their stats page. You can read more about the event here, and check out a trailer for it above.
The Christmas Truce event will last until January 2nd.
Verdun is available on Windows PC, Mac, Linux (all via Steam), Playstation 4, and Xbox One.