This is what the HBS admission blog had to say:
HBS Admissions blog wrote:
today we sent out our first batch of invitations. There is not a regular schedule for sending these out — several days may go by before we send another group. It's also worth repeating that there is no plan or pattern to the sequence of invitations.
Well, they say they have sent out their first
set of invitations. God only knows how may of these
sets they have.