Moed Katan, Chapter Two, Mishnah Five
The final mishnah of this chapter deals with people whose work might be necessary during the festival. The general rule is that while they may engage in their work, they should do so in as private a manner as possible.
1) They may cover [drying] figs with straw.
a) Rabbi Judah says: they may even be pile [the figs] up [in heaps].
2) Sellers of produce, clothing and [other] vessels may sell privately for the requirements of the festival.
3) Trappers [of fish and birds], groats-makers and grist-millers may engage in their work privately for the requirements of the festival.
a) Rabbi Yose says: they were strict upon themselves.
Section one: Drying figs were covered with straw so that they wouldnt get dirty. The mishnah permits one to cover them with straw on the festival, because if they are left uncovered, they might get ruined.
Rabbi Judah says that one may even heap them up together in order to preserve them better.
Section two: The people referred to in this section are selling items that the public might need for the festival. The mishnah says that they may do so, but that they should try to sell in as private a manner as possible. The rabbis wanted to allow people to buy items necessary for the festival, but they also wanted to prevent the week from turning into business as usual.
Section three: Similarly, people who engage in food production may work, but only in a private manner. Groats-makers pounded grain to make it into cereal whereas grist-millers grind it to break it up into small kernels. Since people need these foods during the festival, and they cant be done much ahead of time (they had little means to preserve food), they may continue to work during the festival, as long as they try to be as private about it as possible.
Rabbi Yose adds a fascinating note. He says that people in these professions were strict upon themselves and didnt engage in this work at all. In other words, the previous clauses intention was not to tell these workers that they had to do their work in private, but rather to emphasize to them that they were allowed to work.