1. Fasting on day of Milah:[1]

Letter of law: Some Poskim[2] rule that the father of the newborn is forbidden to eat a meal on the day of the circumcision until the circumcision takes place, as is the law regarding any other time-dependent Mitzvah. However, the mother of the newborn is permitted to eat as normal prior to the circumcision.[3] Furthermore, some Poskim[4] rule that the above prohibition even to the father of the newborn only applies if he himself will be doing the circumcision, however, if he has appointed a Mohel to do so, then it is permitted for him to eat as usual prior to the circumcision. However, in such a case, some Poskim[5] rule that the Mohel is prohibited from eating a meal prior to the circumcision unless the circumcision is not taking place on time. Other Poskim[6] argue on all the above and rule that there was never a prohibition decreed against eating a meal prior to doing the circumcision even for the father the child, and even if he himself is the Mohel. Practically, the custom of many is at the very least not to eat a meal prior to the circumcision even if they appointed a Mohel.[7] However, to eat a snack prior to the circumcision is permitted according to all opinions.

Custom and act of piety: Despite the above letter of the law allowance [to eat a snack and according to many even a meal], it was customary of the original Chassidim [in times of Rishonim] to fast on the day of any Mitzvah which comes on occasion until the actual Mitzvah is performed.[8] Accordingly, some are accustomed for the father of the newborn to fast until after the circumcision.[9]

Sandek: According to all, a Sandek does not need to fast on the day of the Bris. Nonetheless, if the Sandek desires to do so due to Chivuv Mitzvah, then he may.

[1] See Pesakim Uteshuvos 262:6 footnotes 39-43

[2] Nachlas Tzevi 262:1; Daas Torah of Maharsham 262; Peri Hasadeh 3:75; Poskim in Pesakim Uteshuvos 262:6 footnote 39

[3] Nachlas Tzevi ibid

[4] Nachlas Tzevi ibid; See Poskim in Pesakim Uteshuvos 262:6 footnote 40 for dissenting opinions

[5] Nachlas Tzevi ibid; See Pesakim Uteshuvos 262:6 footnote 40 in name of Betzel Hachochma 4:60 for dissenting opinions

[6] Maharam Shick O.C. 287; Yad Shaul 265; Pesakim Uteshuvos 262:6 footnote 41

[7] Birurei Halachos 48:8

[8] Rokeiach 353

[9] Mamar Mordechai O.C. 468:1; Daas Torah of Maharsham 262 in name of Besamim Rosh 74; See Pesakim Uteshuvos 262:6 footnote 41 and 43