For when you do not know what to say. It is the only prayer in the Gospels that Jesus gives in answer to a request — teach us to pray — and it has been the first prayer most Christians learn ever since.
Seven requests, and the first three are not about you. Hallowed be thy name. Thy kingdom come. Thy will be done. Only then bread, forgiveness, protection. The order is the lesson: it puts God's business before yours and then does not apologize for yours.
One line in it is a condition, and it is the only one. As we forgive those who trespass against us. You say it every time, and every time you are agreeing to something.
Galilee, the first century, from his own mouth — and in two forms, because the two evangelists give it differently. Matthew's is longer and is the one we say. Luke's is shorter and comes with the story: the disciples watched him pray and one of them asked him to teach them. He probably taught it more than once.
Three things in the Douay text above are worth knowing. Supersubstantial bread is Jerome's Latin for a Greek word that occurs nowhere else in the language, and nobody is sure what it means; Luke has daily and that is what stuck. Debts is what both Gospels actually say; trespasses is William Tyndale's English of 1526, and though he was executed for that translation, his word is the one every English-speaker says. And the ending you may know — for thine is the kingdom and the power and the glory — is not in the Vulgate, not in the Douay, and not in the oldest Greek manuscripts. It was added by early Christians who found the prayer ended too abruptly. That is why Catholics stop at deliver us from evil and Protestants go on.
The Didache, a church manual written around the year 100, tells Christians to say it three times a day — so within seventy years of the crucifixion it was already a habit with a schedule. Tertullian, a century later, called it a summary of the whole Gospel.
It opens the Communion rite of every Mass. It is the first bead of every decade of the Rosary. It is in the morning and evening sets here, and in Vespers. There is no Christian prayer said more often, in more languages, by more people, and it is the one he gave.