Learning the laws of Aveilus:
It is a widespread custom for one whose parents are alive to avoid studying the laws of mourning. It is thus only studied by the community Rabbi [or potential Posek], for him to answer questions to those who ask him on this subject. On the other hand, some Poskim encourage the study of the laws of mourning, calling it a “Meis Mitzvah” of which it is a great Mitzvah to break the custom of abstinence and study it. Likewise, some Poskim learn that the entire avoidance of studying the subject of mourning is only in public, such as in the Yeshiva, however, to study it in private is not to be avoided. Practically, one who desires to learn the subject but fears from the danger, is to Daven to Hashem prior to commencing the study that no damage occur to them. Likewise, it is to be studied discreetly without publication. Certainly, Rabbanim who must rule on the laws of Aveilus are to ignore the above custom and study the laws in order to be able to give accurate rulings to those who ask them questions. The Rebbe encouraged Rabbanim to write and publicize Halachos on this subject for the sake of preventing people from transgressing. Some write that a most opportune time to study the laws of Aveilus is in the month of Marcheshvan which lacks any Holidays, and is the month in which Sarah Imeinu died.
Studying Tractate Moed Katan: Some avoid studying the Tractate of Moed Katan due to it containing the laws of Aveilus. Practically, the above widespread custom to avoid learning the subject of mourning only applies to the study of the practical Halachos of mourning. However, there is no widespread custom to abstain from studying the Tractate of Moed Katan, and on the contrary many communities study the entire Shas in order, without skipping any Misechta, including Moed Katan.
 See in length Nitei Gavriel Aveilus Hakdama
 Sefer Chassidim 261 calls it a Meis Mitzvah; Letter of Rav Zalman Shimon Dworkin, printed in Koveitz Zalman Shimon p. 64 “It is a widespread custom amongst all Jewry for one who has parents to avoid studying the laws of mourning. It is thus only studied by the community Rabbi [or potential Posek], for him to answer questions to those who ask him on this subject.”
The reason: As they fear it can cause danger, as the verse in Mishlei 3:11 states that one should not despise rebuke, and one may hurry through the study the Tractate [and bring on this danger]. Furthermore, whatever people consider to be of danger, in truth can cause danger simply due to the bad eye they give to it. [See Sefer Chassidim ibid]
 Rav Zalman Shimon ibid
 Sefer Chassidim 261 regarding Moed Katan “Love the Mitzvah which is similar to a Meis Mitzvah being that no one occupies themselves with it…such as if you see that people in your city avoid studying Moed Katan, you shall learn it and receive much reward corresponding to them all, as it is like a Meis Mitzvah. Love the Tractates and laws that people avoid. Tractate Moed Katan is similar to a lone daughter who is left unmarried due to her occupation as a seamstress for burial garments. Moed Katan approached Hashem and asked Him as to why people do not study the tractate and Hashem replied that it is good to study it.”; Yosef Ometz p. 270 writes against skipping the laws of Aveilus.
 Kneses Hagedola Y.D. 245:3
 Sefer Chassidim ibid
 Nitei Gavriel Hakdama
 Maaneh of Rebbe to Rav Gavriel Tzinner regarding his book on Aveilus
 Kemach Soles p. 154
 Sefer Chassidim ibid
 Letter of Rav Zalman Shimon Dworkin, printed in Koveitz Zalman Shimon p. 64