Moses spoke to the heads of the Israelite tribes, saying: "This is what the Eternal has commanded: If a householder makes a vow to the Eternal or takes an oath imposing an obligation on himself, he shall not break his pledge; he must carry out all that has crossed his lips." - Numbers 30:2-3
These are the words that Moses addressed to all Israel on the other side of the Jordan. - Deuteronomy 1:1
Tishah B'Av is a day of mourning the destruction of both ancient Temples in Jerusalem.
I pleaded with the Eternal at that time, saying, "O Eternal God, You who let Your servant see the first works of Your greatness and Your mighty hand, You whose powerful deeds no god in heaven or on earth can equal! Let me, I pray, cross over and see the good land on the other side of the Jordan, that good hill country, and the Lebanon." - Deuteronomy 3:23-25