There is an old custom to fast on a yahrzeit for a parent. In fact, the Rama’s language (OH 429) indicates that in his opinion, this practice goes back at least to the time of the Gemara, where we find the case of a man taking a vow not to eat meat or drink wine just as on the anniversary of his father’s passing (Nedarim 12a). While this is not the only way to understand that discussion in the Gemara, it is interesting to consider why this custom should have come about at all. Why fast on yahrzeit?
The Rosh points out that it is normal to be sad on such days, as it indeed is. Yet, this still begs the question. Why fast?
The Rambam explains in his Laws of Fasts that it is “among the ways of repentance” to fast in response to tragedies. We do so both in order to arouse mercy from heaven and in order to better ourselves. In general, we always fast for these reasons and it stands to reason that the same holds on yahrzeits. These are days that something terrible happened and we seek to do teshuvah and arouse mercy from heaven. Thus, these days are meant to be days of reflection and introspection, not only days of sadness, nostalgia, and appreciation. With that in mind, a yahrzeit becomes a mini Yom Kippur, a day of pain in a very real sense. Yet also a day of elevation of mind, heart, and soul.