Q. A few questions concerning the fast of chosson kallah on the wedding day. Is there any source for the parents also fasting? If the chupa is at night well after tzeis do they still have to fast?

A. Darkei Moshe ( O.H. 580: 1) mentions this tradition, so does Shulchan Hoezer (6: 5), Likutei Mahaiach (H. Nisuin), Sefer Haminhagim (p. 75 quoting Rayatz), Kitzur Nachlas Shivo (148) and others.

Tiferes Leshivo (on ibid.) offers a reason, following Mahari Bruno’s motive for the fast. The Talmud (Shabbos 130a) teaches: “There is no kesuvo (or wedding) where there is no dissension”. To avoid and mend any such discord, since it is the parents who most likely would be the subject or cause to the above, they are then the ones that also fast and pray.

However, Divrei Yatziv (E.H. 5: 74) mentions the reason above for exactly the opposite effect, mainly; that the parents should not fast least they became surly and dour and thus generate more of the unwelcome dissent.

Mogen Avrohom (O.H. 573: 1) mentions that when the father is the one receiving the kidushin on behalf of his daughter, he should fast. This ensues if reason for fasting is to avoid inebriation. Though, if the rationale for fasting is the forgiveness of sins, then he should not fast.

It is interesting to mention that Kedushas Levi (Chaye Sarah) mentions that Eliezer fasted when he gave kidushin to Rivka as an agent of Itzchak. (“I will not eat until I have spoken my words.” (Bereshis; 24:33)

Curiously, Seder Eirusin Venisuin L’rabboseinu Horishonim quotes in the name of Rabenu Ephraim, that there is a tradition where also the other relatives fast, and it is purposely to avoid complete joy (after the churban), or as the Degel Machane Ephraim (p. Bo) indicates that there is forgiveness of sins not only for the chosson and kallah, but also for other relatives that accompany them.

Even though there are many who follow this minhag, it is not widespread or common.
Horav Shlomo Miller Shlit”a advises all to follow their own family traditions, he mentioned that he indeed follows the custom of parental fasting.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a