Pesachim 54 - What happened after Shabbat
Right after the first Shabbat, Adam was granted an extra measure of understanding, and he created fire. To commemorate this, people light a candle with multiple wicks and enjoy its light. The Talmud discusses ten things that were created before Shabbat, such as the well of Miriam, the manna, the script, the mouth of Balaam’s donkey. Incidentally, seven things are concealed from people, among them the day of death, the day when one’s depression will lift, the knowledge of other’s thoughts, what business one will be successful in, and when the Messiah will come.
Continuing with the customs that one should observe together with the local residents, in some places there was a custom to abstain from work on Tisha B’Av , in order to concentrate on mourning, and if this is the custom, one should follow it. Only distracting labor, requiring significant exertion is forbidden. If the place does not have such a custom - this is valid also. However, the Sages should abstain from work on Tisha B’Av regardless of the local custom, because they should feel the loss of the Temple more keenly than the rest.
If one wants to behave like a Sage on Tisha B’Av, abstaining from work, this is permissible, and there is no haughtiness there - so says Rabban Shimon ben Gamliel. And indeed, if an onlooker sees him idle on that day, he will not necessarily conclude that the person is trying to assume the honor due to a Sage. Rather, he will think to himself, "He just does not have work. See, how many idlers are out there in the street."
Art: Death by Jacek Malczewski