The Memorial Museum of the Battle of Normandy in Bayeux provides the most accessible overview of the D-Day landings and the subsequent Normandy campaign of summer 1944. It covers the strategic context (why Normandy, why June 1944), the planning of Operation Overlord, the six Allied landing beaches, and the grinding 77-day campaign that followed before the German breakout and liberation of Paris.
The museum has a good collection of uniforms, vehicles, weapons, documents, and photographs. Visiting here before heading to the beaches makes the individual sites much more comprehensible. An hour is sufficient. The British and Commonwealth war cemetery is directly across the road — 4,648 graves, the largest British Second World War cemetery in France.