Jean Tatlock committed suicide in January 1944. The Oppenheimers second child (a daughter nicknamed "Toni") was born in December 1944. In the episode these two things happen on the same day...
When the D-Day landings are announced on the radio, Frank talks about German V-2 rockets. D-Day was on June 6th 1944, but the Germans only started using the V-2 in September of that year.