Mo'ed Katan or Mo'edh Qatan (Hebrew: מועד קטן , lit. "little festival") is the eleventh tractate of Seder Moed

Moed

Moed is the second Order of the Mishnah, the first written recording of the Oral Torah of the Jewish people . Of the six orders of the Mishna, Moed is the third shortest. The order of Moed consists of 12 tractates:# Shabbat: or Shabbath deals with the 39 prohibitions of "work" on the Shabbat...

of the Mishnah

Mishnah

The Mishnah or Mishna is the first major written redaction of the Jewish oral traditions called the "Oral Torah". It is also the first major work of Rabbinic Judaism. It was redacted c...

 and the Talmud

Talmud

The Talmud is a central text of mainstream Judaism. It takes the form of a record of rabbinic discussions pertaining to Jewish law, ethics, philosophy, customs and history....

. It is concerned with the laws of the days between the first and last days of Passover

Passover

Passover is a Jewish holiday and festival. It commemorates the story of the Exodus, in which the ancient Israelites were freed from slavery in Egypt...

 and Sukkot

Sukkot

Sukkot is a Biblical holiday celebrated on the 15th day of the month of Tishrei . It is one of the three biblically mandated festivals Shalosh regalim on which Hebrews were commanded to make a pilgrimage to the Temple in Jerusalem.The holiday lasts seven days...

 (as both of these festivals are a week in length). These days are also known as "Chol HaMoed

Chol HaMoed

Chol HaMoed, a Hebrew phrase meaning "weekdays [of] the festival" , refers to the intermediate days of Passover and Sukkot. During Chol HaMoed the usual restrictions that apply to the Biblical Jewish holidays are relaxed, but not entirely eliminated...

" days. Mo'ed Katan also discusses the laws of Aveilus (Bereavement). Consisting of only three chapters, it has a Gemara

Gemara

The Gemara is the component of the Talmud comprising rabbinical analysis of and commentary on the Mishnah. After the Mishnah was published by Rabbi Judah the Prince The Gemara (also transliterated Gemora or, less commonly, Gemorra; from Aramaic גמרא gamar; literally, "[to] study" or "learning by...

 ("Completion") from both Babylonian Talmud

Talmud

The Talmud is a central text of mainstream Judaism. It takes the form of a record of rabbinic discussions pertaining to Jewish law, ethics, philosophy, customs and history....

 and the Jerusalem Talmud

Jerusalem Talmud

The Jerusalem Talmud, talmud meaning "instruction", "learning", , is a collection of Rabbinic notes on the 2nd-century Mishnah which was compiled in the Land of Israel during the 4th-5th century. The voluminous text is also known as the Palestinian Talmud or Talmud de-Eretz Yisrael...

.

Permitted Activities on Chol HaMoed

The Mishnah Berurah

Mishnah Berurah

The Mishnah Berurah is a work of halakha by Rabbi Yisrael Meir Kagan , also colloquially known by the name of another of his books, Chofetz Chaim "Desirer of Life."...

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
• Ma'aseh Hedyot- Unskilled work

This list contains many complex details and restrictions which are elaborated on in the Mishnah Berurah.