 Fine weather was forecast for 4 June , with perhaps a morning haze ( between 15 May – 9 June the weather was fair or fine except for 16 , 17 and 29 May , when it was very bad ) . Zero Day was fixed for 7 June , with zero hour at 3 : 10 a.m. , when it was expected that a man could be seen from the west at 100 yards ( 91 m ) . There was a thunderstorm in the evening of 6 June but by midnight the sky had cleared and at 2 : 00 a.m. , British aircraft cruised over the German lines to camouflage the sound of tanks as they drove to their starting points . By 3 : 00 a.m. , the attacking troops had reached their jumping @-@ off positions unnoticed , except for some in the II Anzac Corps area . Routine British artillery night @-@ firing stopped around half an hour before dawn and birdsong could be heard and at 3 : 10 a.m. the mines began to detonate . After the explosions , the British artillery began to fire at maximum rate . A creeping barrage in three belts 700 yards ( 640 m ) deep began and counter @-@ battery groups bombarded all known German artillery positions with gas shell . The nine attacking divisions and the three in reserve began their advance as the German artillery reply came scattered and late , falling on British assembly trenches after they had been vacated . 
