God gifted my wife and I with twins out of which the first to be taken out (caesarian operation) was a boy and after him a girl. Because it wasn’t ’out from the womb’ I couldn’t have a Pidion HaBen - what I would like to know is - which fasts do I have to keep (and which could I keep) for my son. (For instance, do I have to fast the Fast of the First Born of Pessach) Thanks,
The only fast I know of that has to do with a firstborn, is the firstborn fast of Erev Pesach. This fast commemorates the miracle which spared the firstborn Jewish sons from the plague which struck down the firstborn sons of the Egyptians. The custom is that when the firstborn is a minor his father fasts for him until he grows up. The Chok Yaakov is uncertain about the status of a firstborn via caesarian, who is not considered a firstborn regarding inheritance and not in regard to Pidyon Haben (redemption of the firstborn son). Do we say that also regarding the fast he is not considered a firstborn and therefore does not fast, or do we say that only over there he was excluded from the verse, because regarding inheritance it says “Veyaldu Lo Banim” (and they bear him sons – Deuteronomy 21, 15), and regarding Pidyon Haben it says “Petter Rechem” (first issue of the womb – Numbers 18, 15) but over here it is in commemoration of the miracle that they were saved from the death of the firstborn, and he is considered a plain firstborn. In practice it is not required to fast, since it is a great uncertainty. Nowadays however, since people don’t fast anyway but join a Siyum (celebration of the completion of any unit of Torah study) in order to be exempt of the fast; if one wants he may join a Siyum in order to avoid any doubts. [שו"ע ורמ"א או"ח סימן תע סעיפים א – ב, משנ"ב ס"ק א, חק יעקב ס"ק ב, באר היטב ס"ק א, כף החיים אות ג, שערי ימי הפסח להרב יהודה טשזנר שער ב אות א ס"ק ו].