Tractate Moed Katan1 deals mainly with the halakhot pertaining to the intermediate days of the Festival, which are often referred to as simply: The Festivals, in rabbinic literature. The discussion of these halakhot leads to a comprehensive treatment of two other fundamental areas of halakha: The halakhot of mourning and the halakhot of ostracism.
Combining the halakhot of days of rejoicing with the halakhot of days of sadness in one tractate is seemingly contradictory, yet there exists a double connection between the two topics. First, there is a practical halakhic connection between these areas of halakha, as the labors that are prohibited on the intermediate days of the Festival are similar to the labors that are prohibited both during the period of mourning and with regard to one who is ostracized, as he must act as if he is in mourning. It is reasonable, then, to discuss these prohibitions in the same context. Second, there is an intrinsic connection between the halakhot of the intermediate days of the Festival and the halakhot of mourning in that they are derived in a parallel manner.
The essential elements of both the intermediate days of the Festival and the period of mourning are mentioned in the Torah, and some of the broader details are alluded to as well. However, they do not appear as explicit commandments or obligations, and consequently are treated in talmudic literature as rabbinic commandments that are part of the oral tradition. This is the case despite the fact that they are mentioned in the written Torah and were practiced in the time of Moses. Even the Sages who maintain that these halakhot have a basis in Torah law agree that with regard to the specific details, the Bible delegated them to the Sages. As a result, the halakhot discussed in this tractate are not based on the objective categories that characterize Torah law, but like most rabbinic commandments are dependent on circumstances and relative to the situation.
This means that there is a basic difference between the labor prohibitions on the intermediate days of the Festival and those on Shabbat and the first or last days of the Festival. Additionally, these halakhot of prohibited work are not absolute, as their main function is to preserve the unique nature of the day in order to ensure that it is not treated like an ordinary weekday. They therefore include certain leniencies. It is the Sages who determine the criteria of what is prohibited on these days. As there are no comprehensive categories governing the prohibition of labor on these days, the Sages were forced to establish many isolated halakhot that address specific situations.
Due to the fact that so many activities were permitted, the Sages wondered if it would not be better to simply cancel the prohibition of work on the intermediate days of the Festival completely. As these celebratory days did not have any specific ritual obligations, there was concern that they would turn into sole days of idleness. In certain places the Sages in fact drastically limited the scope of the prohibitions of the intermediate days of the Festival.