"Bloody Sunday", when paratroopers shot dead 13 civilians in the Bogside, has already gone into Irish history as a fatally formative moment, a violent occasion which generated much more violence and which convulsed Anglo- Irish relations.
Many events have been organised to commemorate the anniversary, from lectures and debates to tours of the city and a meeting entitled "Secrets and Lies" chaired by Seamus Deane, author of the Booker Prize shortlisted novel Reading in the Dark and a native of the city.
They provide yet another illustration of the cultural and political gap between the Irish and the British. As so often, the Irish insist on dwelling on past incidents the British would much prefer to forget.
Extraordinarily, new evidence and information is still emerging about the events of that bleak afternoon of 30 January 1972. Hundreds of forgotten eye-witness statements have surfaced; so have recordings of the day's Army and police radio messages and other material.
Yet there is still no clear and convincing explanation of how and why it all happened. The civil-rights march that began it all was illegal but in itself largely peaceful, though the inevitable teenagers threw the inevitable stones at troops, in what was par for the course for the city then.
The Paras, possibly the most aggressive unit in the Army and very often at the centre of controversy in Northern Ireland, were sent in. They were new to Londonderry, and were evidently led to expect they might confront IRA gunmen. Within a couple of hours they shot 26 men and youths, killing half of them.
In the aftermath, nationalist Ireland erupted. Bernadette Devlin, the firebrand young nationalist MP at Westminster, ran across the floor of the Commons and flew at the Home Secretary, Reginald Maudling - drawing blood. A huge crowd burned down the British embassy in Dublin. Hundreds of recruits flocked to join the IRA. Loyalist paramilitary groups flourished in the confrontational atmosphere.
Violence mushroomed. In the three years before Bloody Sunday 250 people were killed: in the 11 months after it there were 470 deaths. The second aspect to Bloody Sunday was the reaction of the authorities. The only possible justification for the shootings would have been if the Paras and the IRA had engaged in a firefight, yet there was no sign of this. No soldiers were wounded, no arms or bombs recovered.
The Widgery report into the shootings, by the then Lord Chief Justice, convinced few with its assertion of a "strong suspicion" that many of the dead had handled weapons or bombs.
The Widgery report always suffered from a credibility problem, and time has not been kind to it. The most recent new evidence supports the view that it was aimed not at unearthing the truth but primarily at defending the soldiers' actions.
One of the most potently enduring images of the day was of the priest waving a white handkerchief while helping ferry the injured to safety. He was Father Edward Daly - later to become the Catholic Bishop of Derry.
Dr Daly saw not only the events of the day but also their lasting impact: "A lot of the younger people in Derry who may have been more pacifist before 30 January 1972 became quite militant as a result of it.
"People who were there and saw what happened were absolutely enraged and just wanted to seek some kind of revenge for it. In later years many young people I visited in prison told me quite explicitly that they would never have become involved in the IRA but for what they witnessed, and heard of happening, on Bloody Sunday."
Governments rarely apologise, and the morale of security forces and the continuing republican violence makes it unlikely that this Government, or the next, will agree to some show of public penitence.
But in the absence of such a sign, Bloody Sunday looks set to remain, in nationalist eyes, a historic wrong. According to Dr Daly: "I forgive those responsible for Bloody Sunday - as a Christian I have to. But I do feel that some acknowledgement, an admission of the innocence of these 13 people who were murdered, would help put to rest this awful incident."
n Yesterday 500 people marched through London to mark the anniversary of the shootings, led by Sinn Fein's chief negotiator Martin McGuinness, writes Decca Aitkenhead. Mr McGuinness, who said that Bloody Sunday remained an "open sore" in the minds of Ulster Catholics, confirmed that he would be standing for Parliament in the forthcoming Westminster election, although his party's abstentionist policy would not change and he would not take his seat if elected for Mid Ulster.Reuse content