By coincidence, Sunday’s communal gathering in London to mark the first anniversary of the October 7 pogrom falls — appropriately — on a fast, a national day of mourning.

Tzom Gedaliah, one of the four annual fasts linked to the fall of the Jerusalem and the destruction of the Temple, usually occurs straight after Rosh Hashanah but is postponed a day because of Shabbat. To many, it seems the most obscure of the quartet because the reason for fasting is not so obvious from the biblical account.

The events that underlie it are recorded in the final chapter of the Second Book of Kings and chapters 41-42 of Jeremiah. Jerusalem has been conquered, the Temple razed and many of the inhabitants sent into exile. King Nebuchadnezzar of Babylon appoints Gedaliah as governor of the remnants of the people left in the land.

Gedaliah is a man of good pedigree, whose grandfather Shaphan was a scribe to the righteous King Josiah and whose father Ahikam saved Jeremiah from an angry mob that wanted to kill the prophet because of his rebukes.

Some of those who had fled from the Babylonian invasion to neighbouring countries were encouraged by Gedaliah’s appointment to return to the Kingdom of Judah. “Do not fear… serve the King of Babylon and all will be well with you,” he reassured them.

But not everyone was happy. According to the Bible, Ishmael ben Netanya arrived in Mitzpah, a little north of Jerusalem, where Gedaliah was based, with a band of ten men and assassinated him along with the governor’s entourage of Judeans and Chaldeans (Babylonians). He then took the rest of the townsfolk captive and marched them off towards the border with Ammon.

Before he could cross, however, he was intercepted by another group led by Jochanan ben Kareach. Jochanan had earlier tried to warn Gedaliah that Ishmael had been sent by the King of Ammon (an adversary of Babylon) to kill him; if the plot succeeded, he warned, the people would lose heart and “the remnant of Judah would be lost”.

Jochanan was able to rescue the people of Mitzpah from Ishmael’s clutches. But fearing Babylonian reprisals for the outrage perpetrated by Ishmael and his men, they sought refuge in Egypt.

Ishmael’s motivation for the murder is not made clear other than that it was because Gedaliah had been put in charge by the Babylonians. Ishmael is described as mizera meluchah, of royal lineage, so perhaps he was piqued that Gedaliah had been preferred to him. Or was he driven by outrage at what he saw as collaboration with the Babylonian oppressors? Whatever stirred him, the Bible describes his action as “evil”.

But there is another question: given the scale of the national disaster that befell the Jewish people, why is it that the particular death of one man is thought worthy of a fast? The rabbis of the Talmud later moralised that the fast was established to teach that “the death of the righteous is equivalent to the burning of the Temple of the Lord”.

It is subsequent commentators who draw out the primary reason. As Jochanan feared, the killing of Gedaliah had a demoralising effect and led people to abandon the homeland. Maimonides observed, “the remaining ember of Israel was extinguished, causing the exile to become complete”. According to Malbim, “because of the killing of Gedaliah, they fled from the land”.

Two and half thousands year later, the story of a political assassination with fateful consequences still echoes ominously. Next year will see the 30th anniversary of the murder of Israeli Prime Minister Yitzhak Rabin by a religious zealot opposed to the Oslo Peace Accords with the PLO. If Rabin had lived, would history have taken a different turn, or would the forces arrayed against compromise have proved too strong?

The tradition of fasting to commemorate historic catastrophe goes back to the Bible, where Zechariah speaks of the fasts of the fourth, fifth, seventh and tenth months — understood to refer to Tammuz 17 (when the Babylonians breached the walls of Jerusalem), Tishah b’Av (when the Temple was destroyed), the Fast of Gedaliah and Tevet 10 (when the city was besieged).

Writing decades after the destruction, Zechariah was the prophet of restoration, who urged the returned exiles to rebuild the Temple. The four fast days, he promised, would become occasions of “joy and happiness”. Therefore, he exhorted the people, “love truth and peace” (8:19).

To understand the force of his exhortation, scroll back to the previous chapter where the prophet suggests that although the people had kept the fasts, their observance was wanting because they had not followed the moral imperatives of the Torah. The pointlessness of ritual without ethics is a familiar prophetic theme.

“Judge truthful justice,” counsels Zechariah. “Show kindness and compassion to one another. Do not oppress the widow or orphan, stranger or poor person.” And perhaps thinking back to the killing of Gedaliah, he ends: “and do not think evil of your brother in your hearts.”

To find out more about Tzom Gedaliah, Rabbi Dr Raphael Zarum is speaking on “The Incongruous Fast: What does Gedaliah have to do with Repentance?” at the London School of Jewish Studies on October 6 at 3pm