For Steam:

Halo: Combat Evolved
Halo 2: Anniversary
Halo 3
Halo 3: ODST
Halo: Reach
Halo 4
Borderlands 3

For Xbox: 

"This weekend’s Free Play Days lineup features explosive titles you won’t want to miss out on. Halo: The Master Chief Collection, Borderlands 3 and Dirt 5 are all available for Xbox Live Gold and Xbox Game Pass Ultimate members to play from Thursday, October 14 at 12:01 a.m. PDT until Sunday, October 17 at 11:59 p.m. PDT."

https://news.xbox.com/en-us/2021/10/14/free-play-days-halo-the-master-chief-collection-borderlands-3-and-dirt-5/