This Halacha is an excerpt from our Sefer
Starting a new activity on Mondays and Wednesdays:
It is customary not to begin a new activity on Mondays and Wednesdays. [Some rule that this only applies for mundane matters and not for matters that involve a Mitzvah. Practically, so is the custom. One may thus get married on Mondays and Wednesdays without worry. One is to however avoid moving on Mondays and Wednesdays. If this is unavoidable, then one is to at least move some items into the home, the day before. Alternatively, one is to bring Torah books, and/or have a Chassidic gathering, at the home on the day before. Some rule that one may be completely lenient regarding starting activities on Monday’s, and one is only to be careful on Wednesdays. The custom of the world today is to be completely lenient in this regard although some segments of Jewry are stringent even today.]
 Michaber Y.D. 179:2 based on Teshuvas Ramban 282  who states “Those that do not begin on ב”ד [Mondays or Wednesdays]…”, Based on Zohar p. 273
 The reason: As on Mondays the Mazal of the moon shines, and the Mazal of the moon is not good omen. [Ramban ibid; Shabbos 156a “One who is born by Mazal Levana” will have difficulties-Banaiy Vesoser”] On Wednesday the Mazal of Shabsai Kochav shines, which is also not a good omen. [Ramban ibid] The Mazal that shines on that day or night rules over everything of that entire day. [Pirkei Direbbe Eliezer 6]
 Ruach Chaim brought in Darkei Teshuvah 179:11
 Ruach Chaim ibid
 Igros Kodesh 15:390 [brought in Shulchan Menachem 6:237]
 Dvar Eliyahu 37 brought in Darkei Teshuvah 179:13
 Ruach Chaim ibid
The reason: As then it is considered done for the sake of a Mitzvah, and is hence allowed.
 Birkeiy Yosef 179 in Shiyurei Bracha 3 in name of Derech Chaim brought in Darkei Teshuvah 179:12
 Heard from Rav Asher Lemel Hacohen
 The custom in Belz is to be very particular regarding this matter. They hence do not marry, get engaged or even start the learning semester on Mondays or Wednesdays.