Mo'ed Katan or Mo'edh Qatan (Hebrew: מועד קטן, lit. "little festival") is the eleventh tractate of Seder Moed of the Mishnah and the Talmud. It is concerned with the laws of the days between the first and last days of Passover and Sukkot (as both of these festivals are a week in length). These days are also known as "Chol HaMoed" days. Mo'ed Katan also discusses the laws of Aveilus (Bereavement). Consisting of only three chapters, it has a Gemara ("Completion") from both Babylonian Talmud and the Jerusalem Talmud.
The Mishnah Berurah sums up the important principles that come out of Mo'ed Katan. In Mishnah Berurah 530:1 it lists the activities permitted on Chol HaMoed:
Davar Ha'Aved- One may do work in order to avoid a loss (i.e. if the work is left to be done after Chol HaMoed, a loss will result)
Tzarchei Moed- Even work which requires skill may be done if it is necessary for the purpose of eating on the Moed. For other activities, only work which does not require skill may done if it is necessary for Chol HaMoed.
Bishvil Poel She'Ayn Lo Ma Yochal- If a person does not have what to eat, it is permitted for them to work.
Tzarchei Rabbim- Activities done for the benefit of the public may be done on Chol HaMoed