The DLC was announced at Blizzcon alongside a cinematic trailer featuring StarCraft II's prominent Ghost soldier:
The trailer teases the "first three new RTS missions". Not only is the expected window of launch there, but there has been confirmation since the trailer that this is the first of a few DLC missions we can expect to see throughout next year. These will be single-player campaigns that flesh out and follow the lore of Nova on a journey that is as of yet, unknown journey.
The first of several single-player mission packs will release sometime in Spring 2016. No exact date has been confirmed for any of the packs yet.