SPECIAL DAYS
The Mishna says that one who relates disrespectfully toward the moados, forfeits his share in Olam HaBa. Bartenura explains that this refers to one who works on Chol HaMoeid or drinks and eats his meals as if these were ordinary weekdays.
The fruit in Rabi Yannai's orchard required harvesting on Chol HaMoeid, and since this was an unexpected monetary loss, he picked them at that time. The following year, other orchard owners postponed their fruitpicking until Chol HaMoeid. Seeing this, he made his orchard hefker and did not pick its fruit, because through him others had been brought to sin, by working during Chol HaMoeid.
Ravina once had an opportunity to make a sale on Chol HaMoeid that would earn him six thousand zuz. He decided to wait until after Chol HaMoeid, and the price soared to twelve thousand zuz, bringing him a significant profit.
The Alter Rebbe rules in the Shulchan Aruch that on Chol HaMoeid there is a mitzva of simcha just as on Yom-Tov itself. Due to this obligation, one's clothing on Yom-Tov and also on Chol HaMoeid should be finer than one's Shabbos clothing.
The Rebbe records in his diary how the Frierdiker Rebbe would wear a silk kapote throughout the entire Chol HaMoeid, as was his minhag on Shabbos and Yom-Tov. The Rebbe himself did the same.
On the third day of Chol HaMoeid Sukkos (תש"מ 1979), when the Rebbe came into shul, he turned to the photographer Reb Levi Itche Freiden and said, “Today is Yom-Tov; today one should wear Yom-Tov clothes.”
CONSIDER
Is Chol HaMoeid a weekday or a Yom Toυ?
At the level of pshat, the two words Chol HaMoeid mean "the mundane days of the festival." At a different level, the Tzemach Tzedek explains that the avoda of Chol HaMoeid is to transform the mundane and weekday (chol) into a Yom-Tov (moeid).
UTILIZING THE TIME
The Talmud Yerushalmi teaches:
Work is forbidden on Chol HaMoeid in order to enable us to devote our time to rejoice and study Torah, not to celebrate and party, chas veshalom.
The author of Sefer HaChinuch writes:
The days of Chol Hamoeid were not instituted for work, but for rejoicing before HaShem. This means gathering in shuls and listening to the sweet words of Torah, learning the halachos of Pesach during Pesach and the halachos of Sukkos during Sukkos.
R. Moshe Ben Machir, a mekubal in the generation after the Arizal, writes in his work Seder HaYom:
One should not think that since he is not working, his time should be occupied with eating and touring. In truth, these days are days of Divine goodwill (yemei ratzon) and were given to Yidden so that they can be free to learn Torah. There is heightened kedusha during these days, as can be understood from the extra korban mussaf and the extra aliya in shul. One should enjoy himself physically but should not forget to attend to his neshama, which should be his primary focus.
The Rebbe explains that since during Chol HaMoeid one is forbidden to do any work, one should study Torah day and night, like someone who has all his work done by others, as explained in Hilchos Talmud Torah of the Alter Rebbe.