In all seriousness, this is mine:
The announcement will come on October 2nd.
REASON: October 4th is thirty days out from Election Day, making a “reasonable” legal recourse (and most of the Mercul Peepul are stupid enough to accept it). But the announcement will be on the 2nd, which is a Friday, and will thus be part of the weekend news dump, allowing the MSM to focus-group the event.
TIEBREAKER: Since it must be a black-female, but the current field is too wide to call but there are specific names being vetted, I’m going with Rice.
For that, they need a white leftist.
I'm betting on Newsom or Cuomo.