The beta for Halo 5: Guardians begins on December 29 and ends on January 18, developer 343 Industries announced during Microsoft's Gamescom press conference. This is a slight shift from the initial beta launch dates of December 27 - January 17.
Things unlocked in the beta will carry over to the full launch of Halo 5. Halo 5 marks a return to the franchise's "legacy of competitive multiplayer," 343 said. The game is due out in 2015 on Xbox One.