D-Day refers to June 6, 1944, the day of the Allied invasion of Normandy during World War II. It marked the start of Operation Overlord and Operation Neptune, the largest seaborne invasion in history, which was crucial in liberating Nazi-occupied Europe. The "D" in D-Day simply stands for "day," serving as a military code for the start day of an operation, rather than standing for a specific word like "decision" or "disembarkation." This designation was used to indicate the day of any major operation, with days before and after counted as D-1, D+1, and so on.

Regarding the specific date June 20 in the context of D-Day or the Normandy campaign, June 20, 1944, was during the extended Battle of Normandy following the initial landing. On this day, Allied forces were continuing their advance into occupied France as part of the broader campaign begun on D-Day. Notably, American forces were progressing toward key locations such as Cherbourg, and intense fighting was ongoing as Germans attempted to regroup and defend their positions.