At 28 minutes past seven the signal came, a flare exploding, then whistles to send troops out of trenches and towards the German lines.
By Jonny Dymond
BBC News, northern France
Gathered around the lip of the largest crater left by British mines, a cluster of poppies, deep in its heart, hundreds of people gathered to watch, remembering the last moments of those who would fall.
Prince Charles paid his respects at the Thiepval monument
Nearly 20,000 were killed on the first day. More than a million would die or be injured by the end of the battle.
Dotted around the fields of the Somme are memorials and cemeteries for New Zealanders, South Africans and the French.
A few hours later, a few kilometres away, Canadians gathered to remember their dead.
The premier of Newfoundland and Labrador, Danny Williams, spoke of the sacrifice of those who came from the province.
"It was an impossible mission, but they ran bravely ahead, soldier beside soldier, friend beside friend, brother beside brother. We went in 800 strong and only 68 were able to answer the roll call the following day."
Then came the largest ceremony of all.
In brilliant sunshine, one of the blackest days of the twentieth century was remembered. The mood was sombre and respectful as thousands gathered under the arched monument of Thiepval.
War veterans in wheelchairs, young couples holding hands, parents pushing prams.
The heir to the British throne, Prince Charles spoke of his wife's loss, and his own. He said how hard it was to imagine the devastation of both France and Britain.
"For the first time in our history we put mere boys into an assault against the bomb and the bullets, and the terrible wire entanglements, equipped with little more than raw courage and a deep trust in their young leaders."
The prince shared a moment with WW1 veteran Henry Allingham
Then followed the act of remembrance, the playing of the Last Post, and the laying of wreaths.
After so many years perhaps these commemorations have become familiar yet their power is undiminished.
Every year now as memory turns into history, the dead for a brief moment live again in the fields of the Somme.