In the preliminary instructions it will be noticed that a party of officers and men were detailed to act under the Divisional Gas Officer. Their special duty was to cover the approach of the infantry by the discharge of a smoke cloud. It was hoped to introduce some element of surprise by occasional discharges of smoke during the preparatory bombardment, and so the Corps ordered that the bombardment should be carried out for a period of five days, and the attack would take place on the sixth. These days would be known as U, V, W, X, Y, and Z days.
“Smoke discharges lasting for a period of ten minutes will take place on the days and at the hours mentioned below. They will coincide with the intense artillery bombardment of the enemy trenches. These bombardments will commence thirty minutes before the smoke, and will reach their maximum intensity during the ten minutes that it is being discharged:
U day, no discharge.
V day, no discharge.
W day from 10.15 a.m. to 10.25 a.m.
X day from 5.45 a.m. to 5.55 a.m.
Y day from 7.15 a.m. to 7.25 a.m.
On Z day the smoke cloud will commence five minutes before zero. On the 46th and 56th Divisional fronts its duration will be as arranged by
divisions. On the 37th Divisional front it will continue for one hour.”
U day was the 24th June, but the whole of the great attack was postponed for two days, so that, instead of having five days of the preliminary bombardment, there were seven.