The conference at which the United States and the United Kingdom finally committed to invading Occupied France (and at which they set a general date for it) was the Tehran Conference. This conference was held from November 28 to December 1, 1943 in Tehran, Iran. The main result of this conference was the promise to open a western front in the European Theater of World War II.
Since Germany invaded the Soviet Union in June of 1941, the great bulk of all land fighting in the European Theater was done by the Soviets. They suffered terrible casualties and devastation as they fought off the German invasion. Because they were so hard-pressed, they badly wanted the Western Allies to invade Occupied France and open another front. If the Allies would do this, it would significantly ease the pressure on the Soviets in the east. However, the US and UK were reluctant to do this until they were sure the invasion would succeed. They wanted to take the time to make sure they had enough men and materiel stockpiled on Britain to successfully invade the continent. At the Tehran Conference, they committed to doing so and they set May of 1944 as the date for the invasion.
Thus, the correct answer to this answer is the Tehran Conference.