Moed Katan, Chapter 3, Mishnah 7. The Mishnah deals with the practices of mourning and the question of which of them are observed on Chol HaMoed and which are set aside because of it.
"Ein korin v'lo choltzin v'ein mavrin ela kerovav shel meis":
"Ein korin" - the tearing of the garment is not performed on Chol HaMoed by everyone, but only by the close relatives. Everyone else is exempt from it.
"V'lo choltzin" - there was a practice of baring the shoulder at the funeral of the deceased: lowering the cloak off the shoulder and exposing the shoulder and the arm. This too is not done on Chol HaMoed except by the relatives.
"V'ein mavrin" - the seudas havra'ah is the first meal given to the mourners when they return from the burial. They do not eat from their own food; rather, others bring them their meal. On Chol HaMoed this meal is served only to the close relatives, those who are sitting shivah.
It emerges that all three of these matters - the tearing, the baring of the shoulder and the havra'ah - are done on Chol HaMoed only by the relatives who are obligated in them by law, and by them alone.
"V'ein mavrin ela al mitah zekufah":
The seudas havra'ah is served on an upright bed, that is, a bed standing in its normal position. During the regular days of mourning it was the practice to overturn the beds, a custom connected to the idea of sitting on a low stool - and it is not observed on Chol HaMoed.
"Ein molichin l'veis ha'avel lo b'tavla v'lo b'iskutla v'lo b'kanon ela b'salim":
The meal is not brought to the house of the mourner on a board or a special tray, nor in an iskutla (a special tray of another type), nor in a kanon (a special basket), but only in ordinary, plain baskets. The reason: so as not to embarrass the poor, who cannot afford such things when they come to bring the seudas havra'ah. This law, it would seem, is not connected to Chol HaMoed itself.
Birkas aveilim and the row of comforters:
"V'ein omrim birkas aveilim ba'moed" - a special berachah used to be recited when people gathered in the street, a form of consolation, and its wording is brought in the Gemara in Berachos and in Kesubos, concluding with: "Baruch menachem aveilim." This berachah is not recited on Chol HaMoed.
"V'ein omdin ba'shurah" - the row is not formed, the row in which the comforters stand in a line and offer their consolation as the mourners pass between them.
"U'potrin es ha'rabbim" - the people are sent home, and they no longer need to accompany the mourners. Matters are conducted in a reduced fashion: the public has fulfilled the mitzvah of consoling, and they disperse.
Some understand that the intent is that there is no need to return and stand in the row and console after Chol HaMoed, since this mitzvah was already fulfilled immediately after the burial during Chol HaMoed itself.