The phrase you are thinking of is “a date which will live in infamy,” from U.S. President Franklin D. Roosevelt’s speech to Congress on December 8, 1941, after the Japanese attack on Pearl Harbor.
Roosevelt used this phrase to describe December 7, 1941, emphasizing that the Pearl Harbor attack was a shocking and disgraceful act that would be remembered negatively throughout history. “Infamy” here means lasting notoriety for something profoundly evil or dishonorable, so the line signals that the date itself would be permanently associated with that infamous attack.
