The new zero hour for the commencement of Operation Astonia was 1745 hours on the evening of the 10th September. In the days prior to the 10th, men of the 49th and 51st Divisions concentrated on the approaches to the port and engaged in exercises of street fighting and house clearing. The ammunition brought up contained a high proportion of grenades and sten gun magazines in anticipation of the close quarters combat of the coming days.
The attack plans were described in detail and at all operational levels from Corps to Battalion and all ranks. The level of planning and coordination was highly impressive and was to pay dividends in the operation. Preparation was enhanced over the 4th and 5th September when both British patrols and members of the French Forces of the Interior (FFI) brought in intelligence from within the fortress that described particular strong-points as well as the deployment of German units within and around the many defensive positions.
In broad terms, the 49th Division were positioned on the left flank poised to advance from the north east and south east of the port. 49th Division, supported by the 34th Tank Brigade, were to capture the northern plateau situated to the west of the Lézarde river and to the south west of Montivilliers then secure a bridgehead on the southern plateau. In this first phase, the 51st Highlanders would advance from the north with the support of 33rd Armoured Brigade to secure a base further to the west on the northern edge of the Forét-de-Montgeon. Later the 51st were to subdue the defensive positions around Octeville-sur-Mer, thereby gaining control of the northern outskirts of Le Havre, whilst the 49th Division were to capture the southern plateau.
In the closing phase of the operation, both Divisions were to exploit opportunities to overcome remaining resistance and push into the centre of the town from the north (51st Division) and from the east (49th Division).
The bad weather turned the ground over which the advance was to take place into a morass. Approaches to the port were also heavily mined, so the initial assault was assisted by a number of adapted armoured vehicles, collectively referred to 'Hobart's funnies'.