Tisha B'Av: Halakhah

Halakhah הֲלָכָה

According to the Mishnah (Taanit 4:6), Tisha B’Av is the date the בֵּית הַמִּקְדָּשׁ (Beit HaMikdash, Holy Temple) in Yerushalayim was destroyed about 2,500 years ago, and then again about 2,000 years ago. It was a special place where people from all over the world, Jewish or not, could go to worship and feel close to God. Because of our sadness about not having this place anymore, we have some practices to help us feel and appreciate what we lost. This is called אֲבֵלוּת (aveilut, mourning).

DO

Sitting on the floor
It’s a sign of mourning to sit on the floor, not a chair. We do this in the evening and morning of Tisha B’Av. But if you have trouble sitting on the floor, you don’t have to (Bah Orah Hayyim 559:1).
Reading Eikhah
This book of Tanakh is one of the five megillot, and it’s about the destruction of the first Beit HaMikdash. We read it publicly in a sad melody. Most communities read Eikhah at night when Tisha B’Av begins.
Saying Kinnot
These are sad poems about the destruction of the first and second Beit HaMikdash, and also other tragedies in Jewish history including the Crusades, the expulsion from Spain, and the Holocaust. There are kinnot for Tisha B’Av night and kinnot for Tisha B’Av morning.

DON’T

No eating or drinking
Adults fast the whole day, from the night before. Kids can participate by not eating their favorite foods or treats (Mishnah Berurah 550:5).
No bathing and washing
The Shulhan Arukh (Orah Hayyim 554:7) says we should not wash any part of our bodies on Tisha B’Av. We are allowed to wash off dirt or germs, though (Shulhan Arukh 554:9).
No leather shoes
The Mishnah (Taanit 1:4) says we shouldn’t wear shoes on fast days. The Shulhan Arukh (554:16) says not to wear shoes with any leather in them. Part of the reason is to make ourselves uncomfortable, so if you're able to wear non-leather shoes that are also a little less comfortable than what you normally wear, that's preferred (Mishnah Berurah 614:5).
No greeting each other
There’s a custom not to say “Hi,” (or “good evening,” or any other kind of greeting) to other people. But, if someone who doesn’t know this rule greets you, you can respond. It’s important not to hurt another person’s feelings, but it’s also important to still show that it’s a sad day by not being overly excited.
No learning Torah
A general rule on Tisha B’Av is that we try to avoid doing things that make us happy—and what is happier than learning Torah?! The only parts of Torah we’re allowed to learn on this day are those with topics related to Tisha B’Av, like Eikhah or the destruction of the Beit HaMikdash (Mishneh Torah, Taaniyot 5:10), or this special edition of Devash.