July 1, 1916, A Glorious Summer Day

"Hunched under their assorted burdens, the infantry moved up to the line. Their excitement was mingled with nervousness, but at least the waiting was over. At long last they would have a chance of having a 'proper go' at the Hun. Despite the bellicose array of weapons dangling about their persons, many Tommies had thoughtfully provided themselves with knuckle-dusters, lengths of chain and even vicious knives as their personal contribution to the armoury of battle. Most had never seen a German face to face but, as if anticipating some street corner brawl, they intended to be ready when they did. The fact was that, in spite of the long months of careful rehearsal, of lectures and training, of preparation and of orders, in the untried ranks of Kitchener's Army there was hardly an officer or man who appreciated the difference between a raid and a general attack."

From "Somme" by Lyn Macdonald

The lads had been moving up for days, humping their various loads through the labyrinth of communication trenches to the front.
At least as close to the front as they could get.
One hundred and fifty thousand men take up a fair bit of space when spread out along a line of only 12 miles (the remaining length of frontage involved, to the South, was held by the French).
One can see that even more space would have been occupied when the realization is made that, due to the previous week's barrage and counter barrage, there was nowhere above ground that was survivable.
Consequently, freshly dug jumping off saps, front-line fighting trenches, communication, reserve and rear trenches, all were packed with men awaiting 0730, Zero Hour.
The primary miscalculation of many that day was the assumption that the 1.7 million shells dropped on the German lines had destroyed the enemy's ability to mount a defense. What Huns survived would be completely dazed and shell-shocked and would thus be unable to bring their machine guns to bear.

Wrong, very wrong. The German system was deep, horizontally as well as in the vertical. Many dugouts were thirty feet deep and thus unfazed by the bombardment. The Fritzes in them didn't get much sleep and were almost certainly groggy as they hauled their Maxims to the surface the moment the shelling stopped. But, that's where any perceived British advantage ended.
The disastrous details started piling up immediately.
First of all, of the seventeen mines dug under the German lines, one was late, not helpful but not a problem.
The mine filmed above, one of the two big ones is notable for two reasons:
First of all, the Hawthorne Ridge Redoubt mine is the only mine explosion of the war that was captured on film - and, due to stupid disagreements among the upper echelons, it was detonated ten minutes early.
Hello. Could a forthcoming attack be made any more obvious?
The Red Tabs were so certain of their plan - and so uncertain of their men - that the troops, each man burdened with up to seventy pounds of equipment, some with rolls of wire and shovels to consolidate any captured German trenches, were instructed to walk slowly toward the front.
Referring back to the crowding mentioned above, many of the troops had to crawl out of trenches far in the British rear, some as far as a mile, and hike forward fully in the open.
Not to worry, there won't be any MG's.
Furthermore, even though the plan had been practiced and practiced, the gunners and the infantry weren't really working seamlessly in their "creeping barrage".
The creeping barrage was a tactic in which the field guns would lay down a curtain of fire a few hundred yards in front of the troops, "lifting" their fire to a few hundred yards farther back as the men advanced. It had been timed and rehearsed and showed every indication of working like clockwork, except for those pesky machine guns.
The Royal Newfoundland Regiment was one which was forced to hike some distance to the actual front. Dating back to 1795, they had been the only North American regiment at Gallipoli and were the only colonial regiment in this initial attack.
At Zero, they climbed out and started walking.
Within minutes the regiment was effectively wiped out - before even leaving the British lines.
Of 801 soldiers, 386 were wounded, 255 were dead and 91 were listed as missing. Every officer who had gone over the top that morning was either wounded or dead. The next day, roll call was answered by only 69 men.
Of the thousands of screwups, little and large, making up this disastrous day, one of the more poignant was the reflective tin triangles affixed to the rear of every man's equipment.
Intended to help the Big Brains in the rear follow the progress of their victorious troops, they instead made being wounded in no-man's-land even more perilous. Any movement back to the British lines called forth a torrent of fire from those "destroyed" machine guns.
At the close of business that first day, of the 150,000 men who'd gone forward, more than 57,000 had been killed or wounded.
19,240 were dead, many killed in the first few hours if not minutes of the offensive making the first day of Somme offensive (AKA "The Big Push") The worst in the history of the British Army.

To the South, the French did somewhat better. At the end of the first day, they'd gained five miles, mostly due to the French's greater experience in virtually every aspect of the attack, especially the creeping barrage.
When the debacle ground to a halt in mud and rain on Nov. 18, the British had gained one inch of territory for every four men dead.