http://www.gameinformer.com/b/news/archive/2014/06/11/halo-5-guardians-beta-begins-in-december.aspx

On Monday Microsoft announced Halo: Master Chief Collection, and apart from remastering the numbered games in the series, it also delivers access to Halo 5: Guardians' beta. That beta begins in December.

Speaking to Joystiq, Bonnie Ross, 343's general manager, said that the beta for Halo 5's multiplayer will begin on December 27 and end three weeks later. No word was given if there will be subsequent beta programs after that point.