The saddest day of the entire Jewish year. The day the Beit Hamikdash was destroyed. This is the second most important fast, to Yom Kippur. None of the fasts push over Shabbat, except for Yom Kippur. We don't sit down on chairs. We don't lay down on beds(Certain heights for chairs, and beds are permitted). We fast from the night before, at sunset.