The diaries of Dorothy Day, co-founder of the Catholic Worker Movement, are published this month. Here her publisher and editor recalls her life, with its mix of traditional piety and radical politics. Day, now on the path to sainthood, has already made a significant mark on the Church

For much of her life, Dorothy Day, co-founder of the Catholic Worker Movement, was regarded as a prophetic if somewhat marginal figure in the American Catholic Church. Her combination of traditional piety and her radical stance on social and political issues posed a paradox for many people. She attended daily Mass, read the breviary, said the Rosary, and considered herself in all religious matters a "loyal daughter of the Church". At the same time she called herself an anarchist and backed up her pacifist convictions with tax resistance and repeated acts of civil disobedience.

Dorothy Day was born in Brooklyn, New York, in 1897 and after dropping out of university worked as a journalist for socialist and pacifist publications. In her childhood she had attended services at an Episcopal church but during her twenties she began attending Catholic services. Her interest in Catholicism continued during her relationship with Forster Batterham, a dedicated anarchist and atheist. The couple lived together for four years but her faith led to quarrels and they finally separated in 1927 when she gave birth to a daughter, Tamar, and converted to Catholicism.

Dorothy founded the Catholic Worker Movement with Peter Maurin, a philosopher and former Christian Brother, 75 years ago next month, in an effort to live out the radical implications of the Gospel. Based in "houses of hospitality" for the practice of the "works of mercy", the movement reflected Day's determination to combine charity with a passion for justice. Their first step was to launch a radical newspaper but before long they began to offer practical help to the homeless, providing shelter and friendship. These initiatives evolved into a national network of Catholic Worker communities. Today there are more than 185 communities around the world.

The publication this month of Day's diaries, sealed for 25 years after her death in 1980, offers a new, intimate portrait of her activities and inner life. The Duty of Delight - a title drawn from a favourite line of Ruskin - covers her journey from the earliest days of the Catholic Worker Movement until the final weeks of her life. In a sense, the movement and Day's lifelong vocation were a response to a question she had posed herself as a child: "Where were the saints to try to change the social order, not just to minister to the slaves, but to do away with slavery?"

Many have long believed that Dorothy Day was the model for a new form of holiness. In 2000 the Vatican lent weight to this opinion when it formally accepted her cause for canonisation and bestowed the title "Servant of God". Her diaries certainly support this cause. Almost every page describes her intense spiritual focus and the discipline of prayer and worship that formed the structure of her daily life. "Without the Sacraments of the Church," she wrote, "I certainly do not think that I could go on." At the same time, the diaries chronicle her sometimes complicated relationship with church authorities, and her capacity to combine great love of the Church with suffering over its sins and failings.

In the annals of the saints, Day's diaries offer something unusual: an opportunity to follow, almost day by day, in the footsteps of a holy person. Through these writings we can trace the movements of her spirit and her quest for God. We can see her praying for wisdom and courage in meeting the challenges of her day. But we also join her as she watches television, devours mystery novels, goes to the movies, plays with her grandchildren and listens to opera.

While she was a witness or participant in many of the great social and ecclesial movements of her day, her diaries are a reminder that most of any life is occupied with ordinary activities and pursuits. Inspired by her favourite saint, Thérèse of Lisieux, Day was convinced that ordinary life was the true arena for holiness. Her spirituality was very much focused on the effort to practise forgiveness, charity and patience with those closest at hand.

Like most holy people, she often fell short of her ideals. We know this because she herself calls attention to her faults - her impatience, her capacity for anger and self-righteousness. "Thinking gloomily of the sins and shortcomings of others," she writes, "it suddenly came to me to remember my own offences, just as heinous as those of others. If I concern myself with my own sins and lament them, if I remember my own failures and lapses, I will not be resentful of others. This was most cheering and lifted the load of gloom from my mind. It makes one unhappy to judge people and happy to love them."

Day entitled the story of her conversion The Long Loneliness. Despite her life in community, a certain loneliness remained a constant feature of her life. She writes on one hard occasion, "I have had this completely alone feeling ... A time when the memory and understanding fail one completely and only the will remains, so that I feel hard and rigid, and at the same time ready to sit like a soft fool and weep my eyes out."

In response to the insecurity, the sorrows, and drudgery of life among the "insulted and injured", she tried always to remember "the duty of delight": "I was thinking how, as one gets older, we are tempted to sadness, knowing life as it is here on earth, the suffering, the Cross. And how we must overcome it daily, growing in love, and the joy which goes with loving."

I knew Dorothy Day in the last five years of her life and I have spent many years since then studying her writings. The portrait that emerges in her diaries very much reflects the woman I knew: her interest in the concrete and particular over abstract theories, her love of literature, her eye for beauty, her capacity for joy, her spirit of adventure. And yet in transcribing and editing her diaries even familiar facts stood out in new ways.

I knew, for instance, that she was a woman of prayer. And yet it is striking to see that almost every entry begins with her rising at dawn to attend an early Mass. I knew of her devotion to her daughter and yet it was fascinating to follow her close involvement in Tamar's large family (she had nine children), with whom she moved in for months at a time, rejoicing in the pleasures of her grandchildren, at other times coping impatiently with their adolescent moods.

I knew that she had taken a six-month break from the Catholic Worker Movement during the Second World War. But I was surprised to learn that she seriously contemplated leaving the movement altogether. "What I want to do is get a job, in some hospital as ward-maid, get a room, preferably next door to a church; and there in the solitude of the city, living and working with the poor; to learn to pray, to work, to suffer, to be silent."

The famous turning point in The Long Loneliness was her decision to leave Forster Batterham. I knew she remained in close contact with him throughout her life; he was present at a memorial service at St Patrick's Cathedral, New York, after her death. But I did not know the story of how, in 1959, he begged her to help him care for his longtime companion, Nanette, who was dying of cancer. Without hesitation, Dorothy responded to this appeal, moving in with the couple for several months, keeping them company, nursing Nanette and actually baptising her on the day before she died.

Many of the episodes are well known to readers of Dorothy's writings: her repeated stints in jail as a result of protests against compulsory civil defence drills in the 1950s; her pilgrimage to revolutionary Cuba on the eve of the missile crisis; her fast for peace in Rome during the Second Vatican Council; her arrest at the age of 75 with striking farm workers in California. Even in her last years she undertook new adventures, travelling around the globe, including a visit to Mother Teresa in Calcutta; standing up to threats from the US Government over her refusal to pay federal taxes; even opening a new house for homeless women in lower Manhattan.

But her diaries also document her gradual slowing down owing to age. A heart attack in 1976 stilled her restless travels. In her final months she rarely left her room at Maryhouse in Manhattan. And yet she remained until the end a keen observer, noting the morning glories on her fire escape, and the sight of sunlight on the street outside her window. She rummaged in the "rag bag of memory" for scraps of prayers and lines of Scripture. She had always been a "compulsive" writer, "ever since I was eight years old when I wrote a serial story on a little pad of pink paper for my younger sister's entertainment". And writing was virtually the last thing to go. She wrote until a few days before her death on 29 November 1980 at the age of 83.

Dorothy Day called her newspaper column "On Pilgrimage". There was a literal basis to this, as she often described her far-reaching comings and goings. But it also represented a spiritual attitude, reflected in the words of St Catherine of Siena, which she liked to quote: "All the way to Heaven is Heaven, because He said, ‘I am the Way'."