The Mishna Berura (work by Rav Yisrael Meir Kagan of Radin, 1839-1933), in a very important passage, cites a comment made by Rabbi Abba Bar Mamal in the Talmud Yerushalmi. Rabbi Abba remarked that if he had the support of his colleagues, he would have permitted work during Hol Ha’mo’ed. Even though Halacha imposes certain restrictions on working during the days of Hol Ha’mo’ed, Rabbi Abba nevertheless felt that the Rabbis should have allowed it. He explained that Halacha imposes these restrictions only so that people could spend this time eating, drinking and studying Torah. Instead, Rabbi Abba observed, people spend Hol Ha’mo’ed eating, drinking and engaging in vanity. Rabbi Abba saw that the free time made available by the restrictions on work was being spent on all kinds of unconstructive activities, rather than being used for Torah learning. He therefore felt that it would be preferable to lift the restrictions on work, so that people would at least be productive rather than waste precious time.
Along the same lines, the Mishna Berura cites a comment of the Kolbo (Rav Aharon of Lunel, 1280-1330) that it is a far more grievous sin to engage in frivolity during Hol Ha’mo’ed than to work during Hol Ha’mo’ed.
Therefore, one must utilize his time during Yom Tob and Hol Ha’mo’ed to involve himself in Torah study to a greater extent than he normally does. Those who study Daf Yomi (the daily page of Talmud), for example, must certainly continue their daily study throughout the holiday. In fact, during the holiday they have the special opportunity to spend more time learning, to study the page of Gemara in greater depth than they ordinarily do. This is a precious opportunity that must not be squandered.
The Shulhan Aruch, amidst his discussion of the restrictions on writing during Hol Ha’mo’ed, rules that if a person hears or develops a "Hiddush," a new Torah insight, during Hol Ha’mo’ed, he may write it down, since he might otherwise forget the idea (Orah Haim 546:9). The Mishna Berura, in a fascinating and revealing comment (listen to audio recording for precise citation), claims that this is permissible even if one is confident that he will not otherwise forget the new insight. He writes that at every moment one bears an obligation to study Torah to the best of his ability, which includes arriving at new insights into the Torah. If a person develops a "Hiddush" during Hol Ha’mo’ed and waits to write it down until after the holiday, then he is wasting those moments after the holiday that should be used for developing new "Hiddushim." This would be a misuse of the time which he should be utilizing for arriving at new insights. Writing is permitted on Hol Ha’mo’ed to avoid a loss ("Dabar Ha’abed"), the Mishna Berura notes, and there is no greater loss than losing time from Torah study. Therefore, regardless of whether or not one fears he will forget the idea he learned, he should write it down immediately, on Hol Ha’mo’ed, so that the time after the holiday could be used for developing additional new insights. The Mishna Berura adds that the Bah (Rav Yoel Sirkis, Poland, 1561-1640) wrote his famous commentary to the Tur during Hol Ha’mo’ed.
Therefore, those who attend Torah classes during Hol Ha’mo’ed and wish to take notes may certainly do so.
Summary: The holidays were given for the purpose of celebrating as well as Torah study, and one must therefore ensure to use the free time on Yom Tob and Hol Ha’mo’ed for Torah learning. Although there are restrictions on writing during Hol Ha’mo’ed, it is permissible to write down on Hol Ha’mo’ed new Torah insights that one hears or develops.
