Final answer:
Franklin D. Roosevelt famously referred to December 7, 1941, as the date of Japan's surprise attack on Pearl Harbor, which prompted the U.S. to enter World War II following his 'Day of Infamy' speech to Congress.
Step-by-step explanation:
The significant event associated with President Franklin D. Roosevelt referring to December 7, 1941, is c) The date of Japan's surprise attack on Pearl Harbor. This abrupt military strike led to the United States formally entering World War II. In Roosevelt's "Day of Infamy" speech given to Congress on December 8, 1941, he articulated the treachery of the attack and proclaimed the date as one 'which will live in infamy,' urging Congress to declare war on Japan. The attack sank or damaged numerous U.S. naval vessels, including battleships and destroyed aircraft, with significant loss of life.
Following the attack and the subsequent speech, Congress declared war on Japan with an overwhelming majority. This decision also set the stage for Japan's Axis partners, Germany and Italy, to declare war on the U.S., consolidating America's involvement in both the European and Pacific theaters of World War II. The attack on Pearl Harbor was a pivotal turning point that materially shifted public opinion from isolationism to active engagement in the global conflict.