Taanis, Chapter Four, Mishnah Six. This mishnah teaches us about the things that are forbidden during the week in which Tisha B'Av falls. It is worth noting that many of these practices, according to Ashkenazic custom, already begin from the start of the Three Weeks or from the Nine Days.
"Shabbos she'chal Tisha B'Av lihyos b'sochah":
"Shabbos" here means a week - the week in which Tisha B'Av falls. During that week "asur milsaper umilchabes":
"Milsaper" - one may not cut his hair, and likewise one may not shave, neither the hair of the head nor the beard.
"Milchabes" - one may not launder clothing during that week.
"V'chamishi mutarin mipnei kevod haShabbos" - on the Thursday of that week these things are permitted, out of respect for the approaching Shabbos. This ruling is only relevant when Tisha B'Av falls on Friday, which does not happen according to our calendar; but in earlier times, when the month was sanctified on the basis of witnesses, Tisha B'Av could fall on a Friday. Since Shabbos would arrive right after the fast, on the Thursday before it one was permitted both to take a haircut and to launder clothing, and this is what emerges from the discussion in the Gemara - all out of respect for Shabbos.
The Seudah HaMafsekes:
On Erev Tisha B'Av after midday, and especially at the seudah hamafsekes - the final meal before the fast begins - these restrictions apply:
"Lo yochal adam shtei tavshilin" - one may not eat two cooked dishes, but only a single cooked dish. However, this applies only to foods that are not fit to be eaten raw; foods that are eaten raw are permitted even if one cooked them.
"Lo yochal basar" - one may not eat meat at that meal. We, of course, refrain from eating meat throughout the Nine Days, from the beginning of Av.
"V'lo yishteh yayin" - one may not drink wine.
Rabban Shimon ben Gamliel disagrees and says: "Yeshaneh" - there is no need to refrain entirely; it is enough to make a change from one's usual practice. Someone who normally eats two cooked dishes should eat only one on Erev Tisha B'Av; someone who normally drinks two cups of wine should drink one. It is sufficient that he make some sort of change.
"Rabbi Yehudah mechayev bichfiyas hamitah" - Rabbi Yehudah requires overturning the beds and sleeping on the ground, as is the practice of mourning. However, "v'lo hodu lo chachamim" - the Sages did not agree with him, and there is no need to overturn the beds and sleep on the floor.