Besides the chief feast of St. John the Baptist, that of his nativity, the Church commemorates on the twenty-ninth of August, his glorious death and birth into heaven. Ever since the Fourth Century in Africa, in the East, in Syria, and in many places all over the world, the beheading of St. John has been commemorated on August 29th. When in 362, pagans violated the tomb and burned his remains; some parts were saved by monks, and taken to St. Athanasius at Alexandria. The head is said to have been preserved at Jerusalem, and may later have been brought to Rome.
The Gospel of the Mass recounts his martyrdom. He died because his sanctity as precursor of Christ was intolerable to the immoral Herodias. He anticipated in his death the humiliations of Calvary, but he was rewarded by having his praises spoken by the Savior Himself, even if Jesus and his disciples did not actually assist at his funeral, as some legends maintain.
(Source: The New Roman Missal, Rev. F. X. Lasance)