The shiur deals with the unique halacha of a "taanit chalom" and discusses how this fast circumvents the standard requirement of accepting a personal fast on one's self the previous day and why it is permitted to fast a "taanit chalom" on Shabbat.
This shiur is reposted with permission from the VBM—The Israel Koschitzky Virtual Beit Midrash of Yeshivat Har Etzion