One source says across the street from The Kaiserkeller another source says across from The Indra. In any case, it seems to be within about a block of the future Star Club, on the same side of the street.
One source implies this might be 28 October 1960. Their friend is Helmut (last name?), a waiter they befriended while in Hamburg.
PAUL: "Harald's was on the Große Freiheit. They would serve hamburgers called Frikadellen. We could never understand why they didn't call them 'hamburgers' in Hamburg."
TONY SHERIDAN: "Another place we went to a lot was a typical German "Imbiss" on the Grosse Freiheit called Harold's. The Beatles always ate corn flakes with milk (at one time that seemed to be their staple diet!) - and we all had tea!"
Source: Beatlesource