- Peninei Halakha
The Sages state: "Anyone who works on Tisha Be-Av will never see any sign of blessing from it" (Ta’anit 30b). The reason for this is that working distracts one from mourning. However, the Sages did not prohibit work on Tisha Be-Av explicitly. Rather, some places had a custom prohibiting it, while other places had a custom permitting it. Thus, the Sages ruled that the local custom determines one’s obligation, which means that one may not work on Tisha Be-Av in a place where it is customary to refrain from working (Pesaĥim 54b). Nowadays, the entire Jewish people have accepted the custom to refrain from working on Tisha Be-Av until midday. It is proper to continue doing so after midday, as well, in order to remain focused on the mourning. Therefore, we work after midday only if there is a great need (see sa and Rema 554:22, 24; mb ad loc. 49).
The types of work that are prohibited on Tisha Be-Av are those that take time and occupy one’s thoughts, like sewing, mending clothing, repairing furniture, fixing electrical appliances, and commerce. However, tasks that take little time to complete, like lighting or extinguishing a candle, tying or untying a knot, and traveling for a necessary purpose, are permitted, because they do not distract one from mourning. Writing is also considered work, because it is a distraction, but one may copy over things that are related to Tisha Be-Av.
One may sell food items on Tisha Be-Av, so that people have food for the meal after the fast. At midday, one may begin preparing for the post-fast meal. Some women have a custom to clean the house meticulously after midday, in anticipation of the Messiah, who is supposed to be born on Tisha Be-Av, and one should not denounce their actions (Birkei Yosef 559:7).
A Jew may instruct a non-Jew to do work for him on Tisha Be-Av. However, one may not instruct a non-Jew to do work that is done in public, like building a house or doing business in a store, because it appears disrespectful to the communal mourning (mb 554:46).
One may do work on Tisha Be-Av if delaying the task will cause significant monetary loss, similar to the law on Ĥol Ha-mo’ed (sa 554:23).17
sa implies that the prohibition on working, which is based on custom, applies for the entirety of Tisha Be-Av. The distinction I made between various types of work based on the duration of the task is found in Terumat Ha-deshen and Rema 554:22. The logic behind this distinction is that when it takes time to execute a task, it distracts one from mourning. This is the criterion that should be used to determine the law in every question in this matter. The Aĥaronim debate whether one may write on Tisha Be-Av, as bhl, s.v. "al," and Kaf Ha-ĥayim 554:110 demonstrate. My ruling in the main text follows the fundamental principle of the halakha: that it all depends on whether the task causes a distraction from mourning. If one is concerned that he will forget a novel idea that he thought of, he may write it down, in accordance with the law that permits one to do work in a situation of potential loss.↩︎