See here for an article (
based on an audio webcast according to DoC):
It sounds like what they mostly mean is a date for having a single reservation system. They're not necessarily talking about when they'll announce how points will be earned, how redemption will be determined, how annual elite status will be earned, now lifetime elite status will be earned. We don't know if those will come earlier or later. They seem to be talking about the date that their "backend systems" are merged, which to me sounds like the reservation system most of all. (Right now, of course, you can't make a reservation for a Westin on the Marriott site or a reservation for a Marriott Suites on the SPG site.)
(Admission: I've not listened to the full webcast, only read the linked article so far.)