Microsoft just announced the four games that will be free during the month of May 2016.
On XBOX One, Xbox Live Gold members can download Defense Grid 2 ($14.99) for free during the month of May. Costume Quest 2 ($14.99) will be available as a free download from May 16th to June 15th.
On Xbox 360, starting Sunday, 01 May, GRID 2 ($24.99) will be free for Xbox Live Gold members through May 15th. Then on May 16th, Xbox Live Gold Members can download Peggle ($9.99) for free through May 31st.