Prince William, his wife Kate and brother Harry were set to attend a vigil at the Thiepval Memorial surrounded by the bucolic fields where about a million British, French and German troops lost their lives in the nearly five-month battle.
Guests from several Commonwealth countries and members of the public will hold vigil all night, until the marking at 7:30 am (0530 GMT) of the moment the whistle blew and tens of thousands of troops clambered out of the trenches to be mowed down by German forces.
Some 20,000 British forces died on that first day of the battle, the bloodiest day in the country's military history, while thousands more were maimed and wounded.
“It's quite emotional in a way. I feel it's important to remember these things,” said 73-year-old Irishman William Vernon, arriving at the tiny train station of the village Albert under rainy skies to attend the main commemoration ceremony set for Friday.
Vernon said he was coming to remember his great-uncle — also named William — a banker who died aged 26 in the Battle of the Somme on July 16, 1916. Like the 10,000 members of the public attending, he applied for his ticket online.
Vernon's son — 33-year-old William — said he felt it was important to pay tribute to his relative who died “in the most horrendous conditions. It was an absolutely awful war, a pointless war. To be in the trenches was absolute torture.”
For the first time France and Britain, which fought side by side in the battle, are holding a joint commemoration ceremony at the site.
Beleaguered British Prime Minister David Cameron — mired in a battlefield of his own after Britain voted to leave the European Union — has confirmed he will attend alongside his French President Francois Hollande.
Hollande made a last-minute change to his schedule to attend and show that “beyond what is happening at a European level, the United Kingdom remains a friend with which France wants to keep a relationship,” a source close to the president told AFP.
Prince Charles will also attend the main ceremony at Thiepval on Friday, while 20,000 people are expected at tributes in the Picardy region at six different memorials, according to local authorities.
The Thiepval Memorial is the largest Commonwealth war memorial in the world, commemorating more than 72,000 men who died.
On the Allied side, the battle was seen as a military tragedy not only for the British, but also for their comrades in arms from Ireland, South Africa, New Zealand, Australia and Canada. Officials from those countries will also attend.
Futility of war
The Battle of the Somme, was launched on July 1 as Allied forces hoped to relieve the pressure on the French, who were racking up losses in Verdun by attacking the Germans further north at the Somme.
However carnage ensued on both sides despite the frontline barely moving, in a battle which came to symbolise the horrors of trench warfare and the futility of the war.
One hundred years on, the rolling fields of wheat, maize and sugar beets in the French countryside have become a popular site for “war tourism”, with descendants of the fallen visiting scores of cemeteries where white headstones stretch as far as the eye can see.
Green, overgrown warrens of trenches can still be found, where soldiers once suffered in the thick mud, harassed by rats and haunted by the dead bodies of fallen comrades.
“The rank stench of those bodies haunts me still, and I remember things I'd best forget,” wrote English poet Siegfried Sassoon, who fought in the battle.