Normandy was the location of the D-Day landings on June 6, 1944. In the original series it was an event which lead the Allies one step closer to winning the war.
Wolfenstein 3D: Original Encounter
It's mentioned that (perhaps towards the end of the war) that B.J. worked as an operative and looked for Adolf Hitler from the bunkers at Normandy to Bern, Switzerland.
Return to Castle Wolfenstein
Although anachronistic (the game is set in March 1943), in the OSA briefing room, there is an image with Dwight D. Eisenhower meeting with the men of 101st shortly before the D-Day landings in Normandy (June 5, 1944 or so
The Beach Invasion map is apparently Normandy according to the Strategy Guide.
The background synopsis for some of the missions discuss planned Normandy invasion.
Between 1942-1943, Soviets battle the Axis in the East the remaining Allied forces damage and distract the Axis in the West until an amphibious liberation of Europe can be mounted. The Axis forces were busy fortifying the Atlantic coastline in expectation of Allied raids and an eventual invasion. Meanwhile, the Allies had much work to do before they could hope to mount a successful invasion attempt and had to gain expertise in both beach and parachute landings.
Stefan Kreige wonders if the Allies would ever attempt an invasion of Europe, and he believes it would be where there was a very large stretch of beach. This indicates that the game takes place before Tuesday, 6 June 1944 (seasonally the game is in fall, so it would have to be in Fall 1943 for that to even be possible) however the timeline already diverged from history in the game as one of the blue prints indicated 1944 has already past and it must be 1945 or later.. There are other indications of dates including September 3, 1944, and possibly references as late as 1947 or 1948 on documents.
In reality if it took even a year later for Operation Overlord to commence, Rommel would have completed the Atlantic Wall defenses around Normandy and Europe making a sea attack nearly impossible. Another interesting comment by Stefan is that he mentions that the Allied forces are struggling to win, as well as the Tirpitz attempt on destroying London (and no British air defense to attack it for whatever reason, all seem to indicate an alternate history timeline.
Wolfenstein: The Old Blood
In MachineGames' universe, the D-Day landings failed, leading to the creation of the Normandy Defense Museum in Paris in March, 1946.
The failed Allied invasion of Normandy was in June 1944, spread out on 3 beaches. 156,000 Allied soldiers died during the attempted invasion of Normandy, as compared to only 89 Germans. The failed invasion was subsequently dubbed "D-Day" by the Allied aggressors, where the "D" stands for the English word for "Death". It was perceived by many to be the final nail in the coffin for the already strained Anglo-American alliance.
This is interesting as it would seem to indicate another divergence in the timelines between Wolfenstein and MachineGames universe. As D-Day apparently hadn't happened yet in Wolfenstein, yet taking place in late 1944, 1945 or later.