From August 17 to 24, 1943, British, Canadian, and American leaders met in Quebec to discuss future military strategy. Several things were accomplished at the conference, which was code-named Quadrant.
The allies agreed to begin discussions for the planning of an invasion of France the following May, codenamed Overlord. (“D-Day” did not actually take place until June 6, 1944.)
They also discussed increasing the bombing offensive against Germany and continuing the buildup of American forces in Britain prior to an invasion of France.
Perhaps the most important agreement was that the United Kingdom and the U.S. agreed that neither would use a nuclear weapon – now in rapid development – or communicate nuclear intelligence to a third party without mutual consent.
You can read the agreement regarding nuclear weapons (“tube alloys”) here.