According to an image posted on Reddit, Microsoft will introduce a new promotion that offers free access to select Xbox One games for one day. The name of the promo is reportedly "Free Play Day with Gold," suggesting you get 24 hours of access and that you need an Xbox Live Gold subscription.
The Xbox Support Twitter channels appears to confirm the Free Play Day with Gold promo, suggesting that it will be officially announced soon. It is unclear if there are any restrictions on Free Play Day with Gold titles, or whether or not you're playing the full game or just a demo.
@PrestoFriend531 Gotcha. Looks like it is actually. We'll be in touch here shortly. :) ^AD