We don't know yet when
DVC will begin taking reservations for the Riviera, but here is how DVC handled the initial reservation process for Copper Creek:
Copper Creek sales began on March 8, 2017 to existing DVC members and to new DVC members (no prior ownership) on April 5, 2017. The resort opened on July 17, 2017 (if you are a real Disney geek, you will know the significance of that day).
March 21, 2017: Existing Disney Vacation Club owners who add-on points at Copper Creek can begin booking up to 11 months in advance.
April 19, 2017: New DVC members (no prior ownership) who purchase at Copper Creek can begin booking up to 11 months in advance.
April 26. 2017: Reservations open to all DVC owners, subject to the normal 7 month window. On April 26, non-owners will have access to all available nights from the July 17 opening through November 26 (7 months from April 26.)