Luke 23:17 meaning

The parallel Gospel accounts of this event are Matthew 27:15-16, Mark 15:6-7 and John 18:39

In the course of narrating the events of the third phase of Jesus's civil trial, Luke inserts a brief interjection which helps explain some of the events that follow it.

The three phases of Jesus's civil trial were:

1. Jesus's Arraignment before Pilate (Matthew 27:1-2, 11-14, Mark 15:1-5, Luke 23:1-7, John 18:28-38)
2. Jesus's Audience before Herod Antipas (Luke 23:8-12)
3. Pilate's Judgment (Matthew 27:15-26, Mark 15:6-15, Luke 23:13-25, John 18:38 - 19:16)

Luke's interjection is: Now he was obliged to release to them at the feast one prisoner (v 17).

The word, Now, is Luke's way of indicating that he is interjecting a relevant idea within the flow of his narrative. The pronouns: he and them, respectively, refer to Pilate, the Roman governor of Judea; and the Jewish people. It was Pilate who was obliged to release to them at the feast one prisoner.

Luke's term the feast refers to the Passover holiday which was also called the Feast of Unleavened Bread. The morning of Jesus's trial was likely the morning after the Passover Seder, and was the official beginning of the seven-day Feast of Unleavened Bread—Nisan 15 according to the Jewish Calendar. 

To learn more about Jewish Passover and the Feast of Unleavened Bread, see The Bible Says article: "The Original Passover."

Luke's interjection informs the reader that Pilate was obliged to release to the Jews one prisoner during the Feast of Unleavened Bread. 

Matthew and Mark each make similar interjections in their narrations (Matthew 27:15, Mark 15:6), which describe Pilate's obligation as a custom of goodwill, rather than something that was legally obligatory. Nevertheless, cultural traditions tend to quickly become obligatory expectations. 

Unlike Luke, Matthew and Mark place their interjections between their accounts of the first and third phases of Jesus's civil trial. Luke, who is the only gospel writer committed to put things in their chronological order (Luke 1:3-4) and who is the only Gospel writer to detail the second phase of Jesus's civil trial, is more precise in when and where he interjects this fact. 

Luke makes this interjection after he establishes that the third phase of the trial has resumed and is fully underway (Luke 23:13-16). Pilate has just summarized the events of the first two phases and their conclusions that Jesus is without fault (Luke 23:14-15). 

The Roman governor then made his first attempt to release Jesus by offering to have Him punished (despite having declared His innocence) before he intended to set Him free (Luke 23:16). This offer did not go over well with the chief priests, elders, and scribes who wanted Jesus executed, lest He turn the nation against them, possibly by exposing their wicked and illegal conspiracy to kill Him. 

For his part, Pilate knows Jesus is without fault according to the charges and slanders used against Him (Luke 23:4, 23:14-15, John 18:38b); and he also perceives how they handed Him over because of envy (Matthew 27:18, Mark 15:10). Consequently, Pilate will continually try to release Jesus, until at last, he caves to their evil will. 

The governor's next attempt to release Jesus involves this traditional release of one prisoner at the Feast of Unleavened Bread (Luke 23:18-19). This is apparently why Luke interjects this important fact now.