At 4.10pm the bells of St Eugene's Cathedral in Derry will ring out in honour of the dead of Bloody Sunday.
50 years on from the moment the Parachute Regiment started the fatal firing, the bells will peal 14 times.
Thirteen people were shot dead on the day, a 14th man, who was amongst the many wounded, died several months later.
His family attribute his death to his injuries and his name is included on the official monument.
The sound of the cathedral bells will carry over the city, just as the reverberations of that terrible day rippled down the decades.
The Bishop of Derry Edward Daly understood as well as anyone. As a priest he’d been with his people in the Bogside and narrowly escaped being shot himself.
An image of him waving a bloodied handkerchief as the mortally wounded teenager Jackie Duddy was carried past soldiers became the defining image of the atrocity.
"It plunged the city into grief, and after the grief there was enormous anger," he said in a 1977 interview.
Bloody Sunday was not the start of the Troubles, but after it the annual death toll rose dramatically.
In the city, the sadness and the silence which went with it, was soon accompanied by indignation as the British establishment closed ranks to protect its soldiers.
A tribunal of inquiry which reported months after the killings absolved them of blame.
The Widgery Report, long dismissed as a whitewash, concluded the soldiers had been fired on first and forensics suggested the dead had been in contact with firearms.
The official record had been set.
What followed was a campaign to change that narrative which took almost 30 years.
It took a second public inquiry – the Saville Inquiry – which ran for 12 years and cost almost £200m.
I was in Derry the day its findings were published.
My abiding memory is of the families – brought to the Guildhall to read it ahead of publication – waving copies out the windows at the crowd in the square below to indicate that their loved ones had been vindicated.
Saville concluded that the army had, on balance, fired first; that none of those shot posed any risk to the military; that none of the victims was armed, with the probable exception of one – whose family disputed that finding; and that soldiers had lied to justify opening fire.
The then British prime minister, David Cameron, apologised, telling the Commons the killings had been "unjustified and unjustifiable".
The 50th anniversary of the killings provides a moment of commemoration and reflection.
The families will walk the route of the original march and the names of the dead will be recalled, as they are every year. The Taoiseach will lay a wreath on behalf of the Government.
But there’ll also be exhibitions, lectures, talks, theatre and music. Actor Adrian Dunbar is hosting the commemoration event; President Michael D Higgins is sending a video message to the people of the city.
Lights will be lit, 14 in memory of the dead, one in tribute to the injured, one for all those who lost their lives in the Troubles.
The commemoration’s official title is "Beyond the Silence" to reflect not just what the families endured, but what they achieved in subsequent years.
That commitment to the pursuit of truth which sustained them through the decades is reflected in the President’s contribution to the event.
He’s expected to say that the 30th January 1972 will live long in Ireland’s collective memory, as will the efforts of the families to establish the truth of what happened.
And he’s expected to acknowledge the role of the city and its people in the consensus-building of more recent times.
While the events of Bloody Sunday became a driver of The Troubles, the city was also at the forefront of efforts to find peace.
The principal architect of the Good Friday Agreement, John Hume, was one of Derry’s most famous sons.
Prominent figures played an important role in back-channel negotiations with the British government which led to the IRA ceasefire.
It was here that a template was found for facilitating disputed parades by the loyal orders which was adopted elsewhere.
For some of the bereaved and injured of Bloody Sunday a public acknowledgement of the soldiers’ culpability is enough.
For others, seeing former soldiers face court for what they did remains a goal.
British Army evidence says 21 soldiers fired their weapons on Bloody Sunday accounting for 108 rounds.
After the Saville Report and a police investigation 18 soldiers were considered for prosecution.
Eventually one, Soldier F, was charged with two counts of murder and five attempted murders.
Last July, the Public Prosecution Service said it planned to drop the charges due to concerns about the admissibility of evidence.
That decision is being contested in the courts. Bloody Sunday happened half a century ago, but there are still issues left to resolve.
The families of the dead and injured live every day with the legacy.
The bells of St Eugene’s will peal in commemoration today and the people of Derry will pause to remember.