1925 E. Fraser & J. Gibbons 312 The opening of the main attack of the battle of the Somme was fixed for June 29th, and notified beforehand as ‘Z Day’.
1930 S. Sassoon iv. 61 Operation Orders..notified us that Thursday was ‘Z’ (or zero) day.
1938 E. Blunden In the sour concrete hole the corporal shows his muddy map, his Z Day zone of fire.