To be precise, this should be the "Date of Infamy" speech since President Roosevelt said that December 7, 1941 was a date that would live in infamy. He was, of course, referring to the Japanese attack on Pearl Harbor that happened on that date.
By calling that date a date that would live in infamy, the President was emphasizing the moral justification for US retaliation against Japan. He was stressing that the Japanese had acted dishonorably because they had attacked without a declaration of war. For this reason, the attack was seen as morally wrong.
The "Date of Infamy" speech, then, is the speech in which President Roosevelt denounces Japan's attack and asks Congress to declare war on Japan. When Congress did so, America had officially joined WWII.