The multiplayer beta for Halo 5: Guardians isn't expected to launch for owners of Halo: The Master Chief Collection until December 29, 2014, but according to the Xbox One Preview Dashboard message today, members of the Xbox One Preview program will gain early access to the beta tomorrow.
"Welcome to the next evolution of Halo combat!" the message reads. "As a valued Xbox One Preview participant registered in the program as of this morning, we’re excited to offer you an invitation-only early access opportunity to help us prepare for the public launch of the Halo 5: Guardians Multiplayer Beta that takes place from December 29th – January 18th. This beta is a massive undertaking for everyone at 343 Industries and we’re happy to have you join us for this exclusive event."
The early access period will be available for preview members for two days — beginning December 19th and ending at 11:59pm PST on Sunday, December 21st. Although the full Halo 5: Guardians Multiplayer Beta will feature a total of 7 maps and 3 game types spread out over three weeks, the weekend early access period will only offer content from Week 1 of the beta.
[Reddit]