The history and observance of this minor fast day.
The Fast of Gedaliah is a day set aside to commemorate the assassination of Gedaliah, the Babylonian-appointed official charged with administering the Jewish population remaining in Judah following the destruction of the Temple and exile in 586 B.C.E. It is observed on the third of Tishrei (the day after Rosh Hashanah) with a fast from sunrise to sundown, and like on other fast days, the recital of special prayers ("Anenu") and the reading of selected biblical readings (Exodus 32:14; 34:1-10). In years when Rosh Hashanah begins on Thursday, the fast is postponed until Sunday, as fasts other than Yom Kippur are not permitted on Shabbat.
Seal of Gedaliah
The earliest commemoration of Gedaliah's death might be said to have occurred immediately after his assassination, with the pilgrimage of those who had come to mourn him only to be stricken down by Ishmael.
We next hear of this as a day of mourning connected with three other fast days in the Book of Zechariah. In an oracle dating to the end of the sixth century B.C.E. in Zechariah 7-8, we find a group coming to the prophet to ask whether it is still necessary to solemnly commemorate the destruction of the Temple now that the people have been permitted to return to their homeland and rebuild the Temple. In an extended response that never directly answers the question, but does assure them that God has plans for their future prosperity, Zechariah informs them that these four holidays will one day be celebrated with joy and gladness. Perhaps Zechariah's response was intended to mean that these solemn commemorations were still necessary, but would not be so forever.
It should also be noted that Zechariah makes reference to four fast days associated with the demise of Judah (Zechariah 8:19), but he is only asked about one fast, the one commemorating the destruction of the Temple (Zechariah 7:3). In his response, Zechariah refers to all four fast days. This may suggest that not all Jews observed the other three fast days.
The next reference to the Fast of Gedaliah comes in the Babylonian Talmud. In tractate Rosh Hashanah 18b, the rabbis assign the third day of Tishrei as the date the Fast of Gedaliah is to be observed; the biblical text (Zechariah 7:5; 8:19) simply refers to the month of its observance but not the date. The rabbis add that the fact that a fast day is designated to commemorate Gedaliah's death suggests that the death of a righteous man was just as tragic as the burning of the Temple.