The "Agatha Christie indult" is a nickname applied to the permission granted in 1971 by Pope Paul VI for the use of the Tridentine Mass in England and Wales. "Indult" is a term from Catholic canon law referring to a permission to do something that would otherwise be forbidden.
Following the introduction of the Mass of Paul VI to replace the former rite in 1969-1970, a petition was sent to the Pope asking that the Tridentine Rite be allowed to survive in the dioceses of England and Wales. The petition noted the exceptional artistic and cultural heritage of the Tridentine liturgy, and was signed by many prominent Catholic and non-Catholic figures in British society, including Robert Graves, Professor Sir Maurice Bowra, Iris Murdoch, Sir Kenneth Clark, Cecil Day Lewis, Dame Joan Sutherland, Nancy Mitford, two Anglican bishops, Graham Greene, Yehudi Menuhin - and the crime novelist Agatha Christie. Cardinal John Heenan, the leader of English and Welsh Catholics, subsequently approached Pope Paul VI and asked that use of the Tridentine Mass be permitted.
On 5 November 1971, the Pope granted the request. Between then and the granting of the worldwide "universal indult" in 1984, the bishops of England and Wales were authorized to grant permission for the occasional celebration of Mass in the old form, with the modifications introduced in 1965 and 1967.
English Catholics had a particular attachment to the Tridentine Mass, as the Mass which had been celebrated by the English martyrs of the Reformation and by priests in the years in which Catholicism had been subjected to sometimes severe persecution.
The indult acquired its nickname by virtue of a story told about the Pope's acceptance of the petition:
The story is that Pope Paul read through the letter in silence then suddenly exclaimed, "Ah, Agatha Christie!" and then signed it. He must have been one of her fans. The English Indult of 1971 was granted and thus was saved the old Mass. Ever since it has been known informally as the Agatha Christie indult.
The text of the petition and of the response of the Sacred Congregation for Divine Worship to Cardinal Heenan's request are available online here.