**Gedalyahu** son of Achikam was killed along with the Yehudim who were with him at Mitzpah in year 3339. The Radak wrote [(Yirmiyahu 40)] in Divrei HaYamim that he was killed, and they established a fast on Motzaei Rosh Hashanah because of his death. He wrote the Radak [(Yirmiyahu section 41)] that the fast on the 3rd of Tishrei is because of Gedalyahu, and it appears that Rosh Hashanah that year fell on Thursday, thus the fast was observed on Sunday [(Sha'ar HaDorot section 41)].

The fast of Gedalyahu is on the third, the fast on the fourth is Yom Kippur of the year that the Temple was destroyed, on the fifth day [(Sha'ar HaDorot)].