...in joint doctrine and operational planning.
The official defintion of D-Day in the Dictionary of Military and Associated Terms (Joint Pub 1-02) is: "The unnamed day on which a particular operation commences or is to commence."
This is distinct from C-day, which is: "The unnamed day on which a deployment operation commences or is to commence. The deployment may be movement of troops, cargo, weapon systems, or a combination of these elements using any or all types of transport.
So, a campaign starts with C-Day when forces begin their movement toward their staging area(s). The movement from staging area to the objective commences on D-Day (at H-Hour).
In reality, D-Day has already passed and Brandon is smack dab in the middle of phase 3 (seize the initiative) and maybe phase 4 (dominate) of his campaign. The new HC announcement will actually signify the cessation of tactical operations and the move into phase 5 (stabilize) and finally, phase 6 (enable civil authorities) -- in this analogy, this would be turning over the program's reins to the new HC (phase 5) and allowing him to run the program (phase 6).