It appeared the morning of the sailing. Many were aware of it, but just as many, if not more, were not.
>As an American, he should have known not to sail on Lusitania.
It was not quite as simple as that. Many of the "better placed" Americans were concerned enough before the warning appeared to directly ask Cunard if the voyage was safe. Ogden Hammond, for one, asked a Cunard "high up" (off the top of my head I do not recall whom it was) and was reassured that the voyage would be "as safe as crossing Broadway." Buy a transcript of the Limitation of Liability hearings to read further on this tangent. For those not as wealthy as Vanderbilt or Mrs. Hammond, that is to say 99% of those on board, the timing of the message guaranteed that it could not be heeded. Walking off the ship at that point was possible for all on board, of course, but offloading their already loaded luggage might have been difficult
and only an experienced traveller would have known if the U.S. Line's New York, sailing that day, would have honored their Cunard tickets- for most of those on board the idea of possibly forfeiting their fare would have been a daunting one. As would have been the thought of the 8 day crossing on the New York without one's baggage.
Fact is, " should have known not to" is redolent of the old 'she should have known better' charge people used to make about women who had been the victims of certain crimes. The only person to whom "should have known not to" can be applied in this case is Captain Turner- had he followed the clearly stated Admiralty orders, of which he was aware, it is fairly likely that Vanderbilt and all of the rest would have arrived in Liverpool unscathed. He DID know not to and, for reasons never satisfactorily explained, delivered his ship and close to 2000 people directly to the Germans.
That said, there WERE a lot of cavalier quotes attributed to various
first class passengers by the NY Press on sailing day which can indicate either A) almost bizarre indifference to the obvious, or B) graveyard whistling. If you check out Mike and my article here on ET, there is a long and ingeniously weird rationale for staying aboard the ship offered by Theodore Naish (victim) to his wife, Belle (survivor) quoted in it, which shows the extend that people may have gone to to 'normalise' a decidedly abnormal situation.
One other thought- as of May 1st, nothing of the scale of the Lusitania torpedoing had yet happened, at least with regards to transatlantic travellers, so rationalizing away the ad (which ran on the travel page and not in the hard news section of the paper) would have been easier then than it would have been for someone reading it on May 8th.