From: Mark 6:17-29
John the Baptist Beheaded
 For Herod had sent and seized John, and bound him in prison for the sake of Herodias, his brother Philip's wife; because he had married her.  For John said to Herod, "It is not lawful for you to have your brother's wife."  And Herodias had a grudge against him, and wanted to kill him. But she could not,  for Herod feared John, knowing that he was a righteous and holy man, and kept him safe. When he heard him, he was much perplexed; and yet he heard him gladly.  But an opportunity came when Herod on his birthday gave a banquet for his courtiers and officers and the leading men of Galilee.  For when Herodias' daughter came in and danced, she pleased Herod and his guests; and the king said to the girl, "Ask me for whatever you wish, and I will grant it."  And he said to her, "Whatever you ask me, I will give you, even half of my kingdom."  And she went out, and said to her mother, "What shall I ask?" And she said, "The head of John the Baptizer."  And she came in immediately with haste to the king, and asked, saying, "I want you to give me at once the head of John the Baptizer on a platter."  And the King was exceedingly sorry; but because of his oath and his guests he did not want to break his word to her.  And immediately the king sent a soldier of the guard and gave orders to bring his head. He went and beheaded him in the prison,  and brought his head on a platter, and gave it to the girl; and the girl gave it to her mother.  When his disciples heard of it, they came and took his body, and laid it in a tomb.
16-29. It is interesting that the extensive account of the death of John the Baptist is inserted here in the Gospel narrative. The reason is St. John the Baptist's special relevance in the history of salvation: he is the Precursor, entrusted with the task of preparing the way for the Messiah. Besides, John the Baptist had a great reputation among the people: they believed him to be a prophet (Mark 11:32); some even thought he was the Messiah (Luke 3:15; John 1:20); and they flocked to him from many places (Mark 1:5). Jesus Himself said: "Among those born of women there has risen no one greater than John the Baptist" (Matthew 11:11). Later, the Apostle St. John will speak of him in the Gospel: "There was a man sent from God, whose name was John" (John 1:6); but the sacred text points out that, despite this, he was not the light, but rather the witness to the light (John 1:6-8). More correctly, he was the lamp carrying the light (John 5:35). We are told here that he was a righteous man and preached to everyone what had to be preached: he had a word for people at large, for publicans, for soldiers (Luke 3:10-14); for Pharisees and Sadducees (Matthew 3:7-12); for King Herod himself (Mark 6:18-20). This humble, upright and austere man paid with his life for the witness he bore to Jesus the Messiah (John 1:29 and 36-37).
26. Oaths and promises immoral in content should never be made, and, if made, should never be kept. This is the teaching of the Church, which is summed up in the "St. Pius X Catechism", 383, in the following way: "Are we obliged to keep oaths we have sworn to do unjust and unlawful things? Not only are we not obliged: we sin by making such oaths, for they are prohibited by the Law of God or of the Church."
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland.
Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.