Halo – the flagship game for Microsoft will be free for 7 days according to a recent tweet from Aaron Greenberg – Head of Xbox Games. No other details were released other than the fact it will be free.
— Aaron Greenberg (@aarongreenberg) June 29, 2016
Your guess is a good as mine as to what exactly that means. Strangely there was no time added to the tweet to state when this begins, and at the time of writiing the official store link still shows Halo 5 as a purchase only-
Best off keeping an eye on this if you’re interested in Halo as a free 7 days is good in anyone’s book!