Pesachim Chapter 4, Mishnah 5. This mishnah opens with the question of doing work on Tisha B'Av. On this day a person is meant to conduct himself in a manner of mourning, and therefore the question of work arises. The reference is not to work of the kind forbidden on Shabbos, such as lighting a fire, but to work of the kind forbidden on Chol HaMoed - labor that takes a person's mind off his mourning.
In the words of the mishnah: "B'makom shenahagu la'asos melachah b'Tishah b'Av - osin" - in a place where the custom is to permit doing work on Tisha B'Av, it is permitted to do so. "B'makom shenahagu shelo la'asos melachah - ein osin" - in a place where the custom is not to do work, work is not done.
The root of the distinction between the two customs:
At first glance, if we are speaking of work that takes one's mind off his mourning, it would seem that it should not be done at all. However, some understand that there is a difference between new mourning and old mourning: the prohibition of work applies to mourning that has occurred recently, such as the loss of a close relative, whereas old mourning, such as Tisha B'Av, does not carry with it a prohibition of work.
The law regarding Torah scholars:
The mishnah continues: "B'chol makom talmidei chachamim beteilim" - in all places Torah scholars do not do work. They must be especially careful not to take their minds off the mourning, and also because some understand that it is fitting that Torah scholars feel the loss more acutely.
"Shimon ben Gamliel omer: l'olam ya'aseh adam atzmo talmid chacham" - even in a place where the custom permits doing work, a person should make an effort to conduct himself as a Torah scholar does and not do work. The novelty of the mishnah here is that there is no arrogance in this - there is no concern that he will appear to be putting on airs and behaving above his level; rather, in a matter such as this it is fitting for every person to make himself like a Torah scholar and refrain from work.
Returning to the law of Erev Pesach:
From here the mishnah returns to the matter mentioned in the first mishnah of our chapter - doing work on Erev Pesach before midday, which is the time when the Pesach offering is slaughtered. "V'chachamim omrim: bi'Yehudah hayu osin melachah b'erev Pesach ad chatzos, u'vaGalil lo hayu osin kol ikar":
In Judea: they would do work on Erev Pesach until midday, and only after midday was everyone careful not to do work, since that is the time for bringing the Pesach offering.
In the Galilee: they would not do work at all, and were careful not to do work from the beginning of the day.
It emerges that according to this view, even though the first mishnah stated in a general way that there are different customs in this matter, our mishnah teaches that the custom depended on geography and on the place where a person found himself.
And what is the law regarding the preceding night?
The two differing customs were stated with regard to doing work during the day, and regarding the preceding night there is a dispute:
"BaLaylah Beis Shammai osrin" - Beis Shammai were strict and forbade doing work even on the night before Erev Pesach.
"U'Veis Hillel matirin ad netz hachamah" - Beis Hillel permitted it everywhere until sunrise.
In summary: In this mishnah we learned that doing work on Tisha B'Av depends on the custom of the place, and that the root of the distinction lies in the question of whether the prohibition of work in mourning applies to old mourning as well; that Torah scholars refrain from work everywhere, and that according to Shimon ben Gamliel it is fitting for every person to make himself a Torah scholar, and there is no arrogance in this; and that on Erev Pesach the customs differed according to place - in Judea until midday and in the Galilee not at all - while regarding the preceding night Beis Shammai and Beis Hillel disagreed, the former forbidding and the latter permitting until sunrise.