We can reconstruct things a little bit, anyway. They announced in advance that they were going to have soft openings for the CMs Thursday and Friday nights (the 7th and 8th) from 7 until midnight. That much is obvious from this now-famous backstage poster:
But they ran a full-page ad in the Saturday LA Times, announcing that the HM was open. They had to have purchased the ad the day before at the latest. That means that they already knew before the second night of CM previewing (at the very latest) that they were going to go public the following day. Maybe the first night (the 7th) went so well that they made a spot decision to just go ahead and open to the public on Saturday the 9th. They bought the ad, let the second night of CM previewing go ahead as planned, and the rest is history.