Siman 148 — The laws of the festivals of the idols
Twelve seifim — the Shulchan Arukh announces it itself: ובו י״ב סעיפים. Three days before the festival of idolaters one does not trade with them. What the siman fears is neither the festival nor the trade: it is the sentence the non-Jew will say to his idol while holding in his hand what he holds from us. Eleven seifim measure the distance needed for that sentence not to be said — what keeps, lending and collecting, the gift, one man’s festival, the greeting, the road to the festival — and the twelfth records that it is no longer said (Shulchan Arukh, Yoreh De'ah 148 — 12 seifim)
« שלשה ימים לפני חגם של עובדי כוכבים אסור ליקח מהם ולמכור להם דבר המתקיים » (שו״ע יו״ד קמ״ח:א)
Three days before the festival of idolaters it is forbidden to buy from them and to sell them anything that lasts. One sentence, and the whole siman is in it: it is not the trade that is at issue, but what will remain of it in the non-Jew’s hand on the day of his festival. The eleven seifim that follow only measure that distance.
Shulchan Arukh, Yoreh De'ah 148:1
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Hebrew text of the twelve seifim with a fluent English translation. The three days, what keeps, lending and collecting, the gift and its disposal, the dignitary’s day, the private festival, those who celebrate without believing, the greeting and its doubling, the road to the festival and the permission of our times — explained step by step, with the sugyot, the Rambam, the Tur, the Beit Yosef, the Shach, the Taz, the Baer Hetev, the Pitchei Teshuva and the Nekudot HaKesef.
In-depth pilpul: the chakira between הרווחה and לפני עור, left unresolved in the gemara; Rashi against Rabbenu Tam against the Raavad on what a משא ומתן is; the Shach against the Beit Yosef on the reason for the pledge; the Taz against the Bach on one who gives thanks to an idol, and the Nekudot HaKesef’s reply; and the machloket over the gift one cannot destroy.
A master table of the twelve seifim clause by clause, the five reflex questions that sort the whole siman, the machlokot of the Shach and the Taz side by side, the golden rules, the mnemonic and the classic pitfalls.
The practical halakha according to the ש״ך, the ט״ז, the באר היטב, the פתחי תשובה and the נקודות הכסף, then contemporary cases. Note: the Admur HaZaken did not write a Shulchan Arukh on Yoreh De'ah — a level of psak, not “Daat HaRav”.
But the Shach at once marks its boundary, and it is decisive:
« ומ״מ מה שנזכר בסעיף ט׳ אסור גם האידנא » (ש״ך יו״ד קמ״ח ס״ק י״ב)
And the Rama, having permitted rejoicing with them because of enmity, closes with the בעל נפש: whoever can abstain without enmity keeps away. For application to your own situation, consult your Rav.