Ketubot 5A

משום חשבונות.

It is due to calculations performed on Shabbat to prepare for the wedding. He would thereby engage in weekday matters on Shabbat.

אמר ליה אביי: וחשבונות של מצוה מי אסירי? והא רב חסדא ורב המנונא דאמרי תרוייהו: חשבונות של מצוה מותר לחשבן בשבת! ואמר רבי אלעזר: פוסקין צדקה לעניים בשבת! ואמר רבי יעקב אמר רבי יוחנן: הולכין לבתי כנסיות ולבתי מדרשות לפקח על עסקי רבים בשבת. ואמר רבי יעקב בר אידי אמר רבי יוחנן: מפקחין פיקוח נפש בשבת.

Abaye said to him: And are calculations for a mitzva prohibited on Shabbat? But wasn’t it Rav Ḥisda and Rav Hamnuna who both said: With regard to calculations for a mitzva, it is permitted to reckon them on Shabbat? And R' Elazar said: One may allocate charity to the poor on Shabbat. And R' Ya’akov said that R' Yoḥanan said: One goes to synagogues and study halls to supervise matters affecting the multitudes on Shabbat. And R' Ya’akov bar Idi said that R' Yoḥanan said: One supervises matters of saving a life on Shabbat.

ואמר רב שמואל בר נחמני אמר רבי יונתן: הולכין לטרטייאות ולקרקייאות לפקח על עסקי רבים בשבת. ותנא דבי מנשיא: משדכין על התינוקת ליארס בשבת, ועל התינוק ללמדו ספר וללמדו אומנות.

And R' Shmuel bar Naḥmani said that R' Yonatan said: One goes to theaters [tartiyyaot] and circuses [karkiyyaot] to supervise matters affecting the multitudes on Shabbat, because the fate of the Jewish people or of individual Jews is often decided there and one’s presence could prevent calamity. And the Sage of the school of Menashya taught: One makes matches [meshadkhin] among the families concerned for a young girl to be betrothed on Shabbat, and similarly one may make arrangements for a young boy to teach him Torah and to teach him a craft. Apparently, calculations for a mitzva may be reckoned on Shabbat, including calculations for a wedding. Therefore, this cannot be the reason for the prohibition against marrying at the conclusion of Shabbat.

אלא, אמר רבי זירא: גזירה שמא ישחוט בן עוף. אמר ליה אביי: אלא מעתה, יום הכפורים שחל להיות בשני בשבת ידחה, גזירה שמא ישחוט בן עוף! התם דלנפשיה ⁠— לא טריד, הכא דלאחרים ⁠— טריד. אי נמי: התם אית ליה רווחא, הכא לית ליה רווחא.

Rather, R' Zeira said: It is a decree lest one slaughter a young fowl on Shabbat, due to his preoccupation with the preparations for that night’s wedding feast. Abaye said to him: If that is so, Yom Kippur that occurs on Monday should be postponed when fixing the calendar, due to a decree lest one slaughter a young fowl on Shabbat for the meal on Yom Kippur eve, which is a mitzva. The Talmud distinguishes between the cases. There, with regard to Yom Kippur eve, when one is preparing a meal for himself, he is not preoccupied, and he will not overlook the fact that it is Shabbat. Here, in the case of a wedding, one is preparing a meal for others and is preoccupied. Alternatively, there, on Yom Kippur eve, he has an interval of time during which he can slaughter the bird, as the mitzva is to eat the meal on Yom Kippur eve the next day. Here, he does not have an interval of time, because the wedding and the feast take place at night at the conclusion of Shabbat.