Jeremiah 36:9

They proclaimed a fast— It was customary among the Jews, to proclaim anniversary fasts upon certain days, in memory of some great calamity which had befallen them at that time. Of this kind were the fasts of the 4th, 5th, 7th, and 10th months, mentioned by the prophet Zechariah; the first instituted in memory of the city's being taken by Nebuchadnezzar; the second, in memory of the temple's being burned in that month; the third, for the murder of Gedaliah; and the fourth in memory of the siege which then began. See Lowth, and Zechariah 3:5; Zechariah 8:19.