Gedaliah from Wikipedia
According to the Hebrew Bible, Gedaliah (pronounced /ɡɛdəˈlaɪ.ə/ or /ɡɨˈdɑːljə/; Hebrew: גְּדַלְיָּה G'dalyyah) was appointed by Nebuchadnezzar II of Babylon as governor of Yehud province, which was formed after the defeat of the Kingdom of Judah and the destruction of Jerusalem, in a part of the territory that previously formed the kingdom. He was supported by a Chaldean guard stationed at Mizpah. On hearing of the appointment, the Jews that had taken refuge in surrounding countries returned to Judah.
Gedaliah was the son of Ahikam (who saved the life of the prophet Jeremiah) and the grandson of Shaphan (who is mentioned in relation to the discovery of the scroll of Teaching that scholars identify as the core of the book of Deuteronomy).
Gedaliah was a wise man, gentle and modest. He zealously began to encourage the people to cultivate the fields and vineyards, and thus lay the foundation of security. Many who had fled to neighboring lands during the war of destruction were attracted by the news of the revival of the community. They came to Gedaliah in Mizpah and were warmly welcomed by him.
Among the refugees who had joined Gedaliah in Mizpah was Yishmael, the son of Nataniah, a descendant of the royal house of Zedekiah, the last king of Judah. Baalis the king of Ammon, who had been following with apprehension the regrowth of Judah under its new governor Gedaliah, encouraged and sent Yishmael to assassinate him. In the seventh month (Tishrei), Yishmael came to Gedaliah in Mizpah and was received cordially. Gedaliah had been warned of his guest's murderous intent but refused to believe his informants, believing that their report was mere slander. Yishmael murdered Gedaliah, together with most of the Jews who had joined him and many Babylonians whom Nebuchadnezzar had left with Gedaliah (Jeremiah 41:2-3). The remaining Jews feared the vengeance of Nebuchadnezzar (seeing as his chosen ruler, Gedaliah, had been killed by a Jew) and fled to Egypt. Although the dates are not clear from the Bible, this probably happened about 580 BCE, some six years and three months after the destruction of Jerusalem and the First Temple in 586 BCE. If Mizpah was Tell en-Nasbeh on the Shechem road, Ishmael would not have fled to Ammon via Gibeon  which is located to the West near Neby Samwil ("Tomb of Samuel"), which overlooks Jerusalem. Furthermore, Judas Maccabeus, preparing for war with the Syrians, gathered his men "to Maspha, over against Jerusalem: for in Maspha was a place of prayer heretofore in Israel".
Fast of Gedaliah
To lament the assassination of Gedaliah, which left Judah devoid of any Jews and Jewish rule and completed the destruction of the First Temple, the Jewish Sages established the third day of Tishrei as the Fast of Gedaliah. Although Gedaliah's assassination apparently occurred on the first day of Tishrei, the fast is observed on the third day so as not to coincide with Rosh Hashanah.