Taanit, Chapter Four, Mishnah Seven
This mishnah talks about the week which leads up to Tisha Bav and the day before.
1) During the week in which the ninth of Av falls it is forbidden to cut the hair and to wash clothes but on Thursday it is permitted in honor of Shabbat.
2) On the eve of the ninth of Av one should not eat a meal of two cooked dishes, nor should he eat meat or drink wine.
a) Rabban Shimon ben Gamaliel says: one should make change [his diet.]
3) Rabbi Judah obligated turning over the bed, but the sages did not agree with him.
Section one: Starting in the week in which Tisha Bav falls one begins to mourn by not cutting hair (this includes shaving) or washing clothes. However, if Tisha Bav falls on Friday then it is permitted to cut ones hair and wash ones clothes on Thursday in preparation for Shabbat. In todays calendar Tisha Bav can never fall on Friday or on Shabbat. We should also note that in Ashkenazi tradition these prohibitions begin with the seventeenth of Tammuz, three weeks before Tisha Bav.
Section two: The meal before Tisha Bav is supposed to be a simple meal, one that does not consist of two cooked dishes, nor meat or wine. This sharply contrasts with Yom Kippur, before which one is mandated to have a large meal in celebration of the coming holiday.
Rabban Shimon ben Gamaliel is more flexible with regard to this meal. He just mandates that one change his normal eating habits. If one normally has two cooked dishes, one should have only one. If one normally eats a lot of meat, one should just have a little.
Section three: Turning over the bed is a sign of mourning. In the time of the mishnah it was the custom of mourners to turn over their beds as a sign of the overturning of their worlds which occurred when they lost a relative. Rabbi Judah says that on Tisha Bav everyone should turn over their beds as a sign of the collective mourning of the people. However, the other sages disagree with him.