Introduction to Moed Katan
Moed Katan is about the halakhot governing the intermediate days of Pesah and Sukkot (called Hol Hamoed, which means the non-sacred days of the festival). The Torah says that one cannot do work on the first and seventh days of Pesah, on the first day of Sukkot and on Shmini Atzeret. It doesnt say anything about the intermediate days. One the one hand these days are still part of the festival. Special sacrifices are offered and special prayers recited. On the other hand the Torah does not prohibit work on these days. This leaves their laws somewhat ambiguous. They tend to be quite flexible. The same work that in one situation is prohibited may be in other situations permitted.
According to the rabbis, some work is prohibited on Hol Hamoed but some work is allowed. There are certain general principles that guided the rabbis in deciding which work was allowed and which was not. I shall list these briefly here. Most of our tractate gives examples that lead to these principles.
1) If the work must be done on Hol Hamoed or a financial loss will be incurred, it is generally permitted.
2) If it is very difficult, laborious work, it will likely be prohibited.
3) One should not do work on Hol Hamoed that could have been done beforehand.
Much of the third chapter deals with the laws of mourning. These laws are brought there on account of a mishnah which teaches that one cannot mourn on a festival and that a festival can put an end to the shivah (seven) days of mourning. We will discuss this at much greater length when we get to the third chapter.
Good luck and as always, kol hakavod for continuing to learn.