The most mourning day for Jews is the Ninth of Av (Hebrew תִּשְׁעָה בְּאָב Tish'a be-av). On this day, they do not eat anything, do not drink, do not wear leather shoes. The fast begins on the evening of Av 8, a few minutes before sunset, and ends after the stars appear in the sky on the evening of Av 9.
Day 9 of Av is also the only day of the year when a Jew is not only not obliged, but also has no right to study the Torah (after all, study is considered a source of joy).
Initially, the fast of Av 9 is associated with the “sin of spies” - when Moses led the Jews to the borders of the Promised Land, they were afraid to immediately enter it and begged Moses to send spies so that when they returned, they would describe the country and the prospects for life in it. And although this request was an expression of doubt in God, who brought the Jews out of Egypt and promised them this country, yet Moses agreed to send the heads of the 12 tribes for reconnaissance. The returning scouts reported that the country was "fortified to the sky" and inhabited by giants, against whom the Jews are "small as grasshoppers." Only the heads of the two tribes claimed that the promised land was beautiful and that it was worth entering it. The people believed, of course, the majority, and all night from the eighth to the ninth of Av, the Jews wept, saying that God had brought them to this country out of malice, and that it would be better if they died in the wilderness... Then God became angry and said that in this once the Jews cried in vain, but now they will have many reasons to cry on this night. This will be the punishment for the sin of unbelief.