Current fan estimates suggest the the upcoming update will be released tomorrow if the company is following they previous release pattern however, this has not been officially confirmed by Niantic. According to reports, the update has already been added to the game but Niantic is waiting until Wedsnesday before they release the new features globally.
It’s been confirmed by Niantic that this update will include the new Pokemon Go Buddy System that will allow players to take their Pokemon on walks to earn candy; candy is rewarded to the player for walking the specified distance and each Pokemon has a different KM for the player to walk.
The developer has been quiet about what else will be included in the update but fans are hoping that the recent addition to the games code means that Niantic is getting ready to make legendary Pokemon available for capture in the game. Legendary Pokemon can also be taken on walks so it would not be surprising if the team was preparing to launch them also.