343 Industries is continuing a-pace with development of Halo: Master Chief Collection, and following up from Halo: Combat Evolved, which dropped earlier than expected earlier this month, it seems now that Halo 2 is next, and you won't have to wait long to test it, if you're an insider.
Halo 2 and Halo 2: Anniversary are both expected to enter testing by the end of March. Halo: Reach's Forge and Theater are also expected to enter testing this month, also, according to a blog post by 343 Industries.
There's a chance they might miss the deadline, but it seems to be on course for a test release this month as it stands.
Often things are rough around the edges and truly represent a work-in-progress. Building these games is a process and things can change. All of the content listed above may change and so may the timing, but if all goes well, we are targeting our next public flight for the end of March
You'll need to be in the Halo Insider Program to be able to take part in testing, but also be aware that only selected numbers of insiders are chosen for the earlier testing flights.