Siman 270

The custom is to say the chapter "Bameh Madlikin," because it contains the laws of lighting, and perhaps one acted not in accordance with the law, having lit oils or wicks with which it is forbidden to light—he may go and rectify it. And also because it contains the Mishnah: "Three things a person must say within his home on the eve of Shabbat toward dusk: Have you tithed? Have you made the eruv? Light the candle," and through the recitation, he will remember this.

And according to this, we ought to say it before the Arvit prayer, for after the Arvit prayer it is certainly not in our power to rectify, as we have already accepted the Shabbat upon ourselves with "Barchu." And there are places where they say this after the Arvit prayer, and it is not correct. And there is one who wrote that because we are not accustomed to lighting with prohibited wicks and oils, therefore we are not concerned about this. (Bach) And it seems to me there is another reason: granted, in ancient times when they would pray Arvit while it was still very much day, and for them the acceptance of Shabbat was with "Barchu," it was indeed a rectification for one who had transgressed this. But now that we do not pray so early, and for us the acceptance of Shabbat is with "Mizmor Shir L'Yom HaShabbat" and "Lecha Dodi," if so, even if we say it before "Barchu," we have already accepted the Shabbat upon ourselves and it is impossible to rectify; also, in most places it is after sunset, and therefore they are not accustomed to be exacting in this. And we have already written in siman 267 that there are those among us who do not say "Bameh Madlikin" at all, but rather a passage from the Zohar Terumah, "Kegavna" etc., see there, and it is all for the reason we have explained.

There are those who do not say it on a Yom Tov that occurs on the eve of Shabbat or during Chol HaMoed, and also on a Yom Tov that occurs on Shabbat, and they also do not say the psalms and "Lecha Dodi" then, only "Mizmor Shir L'Yom HaShabbat." For when we say then "let us welcome the presence of Shabbat" – it is a slight to the Yom Tov, as it implies and not the presence of Yom Tov. However, the Sephardim begin from "Mizmor L'David," and everything is according to the custom. And there is one who wrote that we do not say "Bameh Madlikin" on the Shabbat of Chanukah, but the custom is not so. (See Bach and Taz and Magen Avraham)