There was no sense of dread in the Bogside early on that cold bright morning. The no-go area was quiet, the streets almost deserted. A few people bought milk or Sunday papers in the only shop open near the junction of Rossville Street and William Street.
The shadow of a slowly cruising Saracen armoured personnel carrier passed the shop window. Like a dog that has spotted a cat, a small boy - no more than four or five years old - was out the door in a flash and straining to pick up from the pavement a rock the size of his head.
"Come back here this minute, Johnny," his sister screamed at him in rage and panic. "Your mother will kill you." Reluctantly he slunk back to her side.
The images and sounds of the day are etched indelibly on the memory, but disjointed and separate, like a series of snapshots or the frozen frames of a film.
The stillness of the long morning was punctuated from time to time by the familiar earsplitting detonation of a blast bomb slid by youths - purely for shock value - under the occasional prowling Saracen.
An air of anticipation and urgency developed as midday passed and clusters of people hurried towards the assembly point for the banned civil rights march, but the mood was festive rather than foreboding.
In one of the few undisputed observations in his subsequent report, the late Lord Widgery wrote: "The marchers gathered on the Creggan Estate on a fine sunny afternoon and in carnival mood."
A great succession of buses arrived at the grassy square high up above the Bogside and disgorged thousands of passengers in high good humour. The crowd swelled to unexpected proportions and the parade moved off, at first slowly, but gathering pace as it headed down the steep streets towards the Bogside and the centre of the city.
As the throng swept down William Street approaching the main British army barrier, the lorry which led it swung right into Rossville Street and civil rights stewards managed to turn the vast bulk of the marchers to follow it towards Free Derry Corner.
A section, perhaps a hundred or more, remained behind to challenge the soldiers who were preventing them from reaching the city centre. The confrontation that followed was typically intense but short-lived.
Stones and other missiles bombarded the soldiers, who replied with rubber bullets, CS gas and a water cannon. Lord Widgery later quoted the operation order for the day, which stipulated that CS gas was not to be used "except as a last resort if troops were about to be overrun", and he appeared to assume that this had been adhered to.
In fact, the gas canisters rained freely on the rioters from the first moment and the choking clouds became so dense that all but a handful gave up the struggle and began to retreat into the Bogside.
This reporter, too, was gassed so badly that he stumbled blindly away through an alleyway into St Columbcille Court to lie on the ground and try to draw breath.
It was at this point that the paratroopers, in a so-called "scoop-up" operation, raced into Rossville Street and the deadly fusillade of fire was opened. Time went into slow motion and the firing, which continued for perhaps 20 minutes, seemed relentless.
Those of us lucky enough to have been able to take shelter in a flat emerged into a city shrouded in dusk and profoundly permeated by shock and disbelief. Everyone had fled from the darkening streets, to the hospital in search of wounded relatives, to living-rooms to exchange experiences and await the evening news.
The events and days that followed brought a deepening of the horror as the details unfolded. The bereaved were cross-examined by a freshly-arrived media swarm who had not shared the cataclysmic experience and were frequently incredulous and antagonistic.
The avalanche of statements and counter-statements had to be reported; the wounded had to be interviewed in their hospital beds; the funerals, heart-rending in their poignancy, had to be covered. Those who had the distraction of a professional job of work to do were perhaps fortunate.
Along with the profound anger and pain, suspicion and bitterness accumulated as the signs emerged that there would be no official acknowledgement of the truth.
Then came Lord Widgery, to devise - as it turned out - a formal exculpation of the army, and, even worse, to smear "some" of the victims as having been gunmen or bombers.
A long, slow festering began of the psychic and physical trauma that had been inflicted on Derry. That day of such bright and optimistic beginnings was transformed inexorably and - it must be asserted - cynically into a cause and symbol which would fuel a generation of harrowing violence in the North.