In case anyone wants the answer, it's late December 1938, when two physicists went for a walk in the snow near Stockholm to discuss findings a colleague sent via letter.
Eh, I don't even want to give credence to Carlson's garbage by repeating it. But even if we imagine Otto Frisch was somehow there alone making drawings in the snow, we still know when it happened.
Of course we shouldn't do that. Meitner was by far the more experienced scientist and expert on the topic.