Rabbi Chiya bar Abba said in the name of Rabbi Yochanan: When the Mishna said “a son who is of age,” it does not mean that he is literally of age, and when the Mishna said “a minor son,” it does not mean that he is literally a minor. Rather, an adult who is dependent (for support) on his father’s table is regarded as “a minor,” and a minor who is not dependent on his father’s table is regarded as “one who is of age.”
It is brought in the sefer Peninim Mi’shulchan HaGr”a that once when the Vilna Gaon was travelling in Europe, he was hosted by a man well-versed in Torah learning. The man showed the Gaon what his deceased father had written on the margin of his Chumash regarding the verse in Breishis [1:16]: And Hashem made the two great luminaries: the great luminary to rule the day and the lesser luminary to rule the night, and the stars. Written on the margin was the following abbreviation: גועשאנ"ק. Many people had attempted to decipher the meaning of this, but to no avail.
The Gaon took a glance at the word and explained as follows: The abbreviation stands for the following: גדול וסומך על שולחן אביו נקרא קטן - an adult who is dependent (for support) on his father’s table is regarded as “a minor.” The explanation was now self-evident. His father was bothered why the moon was referred to as “the lesser luminary.” The answer was that since the moon has no light of its own, because it has no significant internal source of energy, it is referred to as “the lesser luminary” because its secondary light is produced by what it reflects from the sun.
Rashi cites from Chazal that they were both created the same size, but the moon complained and said that two kings cannot use the same crown and therefore the moon was diminished. The question is asked: It is well known that the moon does not have any intrinsic light source of its own, but rather it is only reflecting the sun light. What is the meaning that they were created equally?
Reb Aryeh Tzvi Frummer answers that that this was precisely the punishment to the moon; the moon did not decrease in size but rather its retribution was that it will not contain its own light and it will only provide light that it receives from the reflection of the sun.
Initially, the sun and the moon were both gedolim since they both had an intrinsic light source; afterwards, the moon became a katan because it could not provide light by itself. The Zohar in Breishis seems to explain in an identical manner.
The Beis Yosef (O”C 31) cites the Zohar in Shir Hashirim that Chol Hamoed is akin to the moon; it does not have its own sanctity but rather it receives kedushah from the Yom Tov.
It is for this reason why Mesechta Moed Katan is called Moed Katan. Since it contains many halachos regarding Chol Hamoed, it is called Moed Katan.