The invasion of France by the Allies on 6 June 1944 was known as D-Day.
Why ‘D’ and not ‘F’ (for France) or ‘I’ (for invasion) or any other letter?
The simple answer is that ‘D’ stands for Date. The US military regularly used this type of shorthand and H-Hour is another example.
Image: By Chief Photographer’s Mate (CPHoM) Robert F. Sargent [Public domain], via Wikimedia Commons