Siman 393
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א אֵין מְעָרְבִין עֵרוּבֵי חֲצֵרוֹת וְשִׁתּוּפֵי מְבוֹאוֹת בְּיוֹם טוֹב שֶׁחָל לִהְיוֹת בְּעֶרֶב שַׁבָּת. וְאִם שָׁכַח וְלֹא עֵרֵב מֵעֶרֶב יוֹם טוֹב, וְחָלוּ שְׁנֵי יָמִים טוֹבִים בַּיּוֹם הַחֲמִישִׁי וְיוֹם שִׁשִּׁי, יְעָרֵב בְּיוֹם טוֹב עַל תְּנַאי שֶׁיֹּאמַר: אִם הַיּוֹם חֹל יְהֵא זֶה עֵרוּב, וְאִם הַיּוֹם קֹדֶשׁ אֵין בִּדְבָרַי כְּלוּם. וּלְמָחָר יֹאמַר: אִם הַיּוֹם קֹדֶשׁ הֲרֵי עֵרַבְתִּי מֵאֶתְמוֹל, וְאִם הַיּוֹם חֹל יְהֵא זֶה עֵרוּב. וְהָנֵי מִלֵּי בִּשְׁנֵי יָמִים טוֹבִים שֶׁל גָּלֻיּוֹת, אֲבָל בִּשְׁנֵי יָמִים טוֹבִים שֶׁל רֹאשׁ הַשָּׁנָה, לֹא, דִּכְיוֹמָא אֲרִיכְתָּא דְּמַיָּא. וְעַיֵּן לְקַמָּן סי' תקכ"ח.
1 One does not make eruv chatzeirot (courtyard eruv) or shitufei mevoot (alleyway partnership) on a festival that falls on Friday. But if one forgot and did not make an eruv before the festival, and two festival days fall on Thursday and Friday, one may make an eruv on the festival conditionally, saying: "If today is a weekday, let this be an eruv; if today is holy, my words are nothing." And the next day one says: "If today is holy, I already made an eruv yesterday; if today is a weekday, let this be an eruv." This applies to the two festival days observed in the diaspora, but not to the two days of Rosh Hashanah, for they are considered like one long day. (See below, section 528.)
ב אֶחָד עֵרוּבֵי חֲצֵרוֹת וְאֶחָד שִׁתּוּפֵי מְבוֹאוֹת, מְעָרְבִין אוֹתָם בֵּין הַשְּׁמָשׁוֹת, וַאֲפִלּוּ אִם כְּבָר קִבֵּל עָלָיו תּוֹסֶפֶת שַׁבָּת. וְיֵשׁ אוֹסְרִים אִם קִבֵּל עָלָיו תּוֹסֶפֶת שַׁבָּת.
2 Both eruv chatzeirot and shitufei mevoot may be established during twilight, even if one has already accepted the addition to Shabbat. Some authorities forbid this if one has already accepted the addition to Shabbat.
ג עֵרֵב לִשְׁנַיִם, לְאֶחָד מִבְּעוֹד יוֹם וְנֶאֱכַל הָעֵרוּב בֵּין הַשְּׁמָשׁוֹת, וּלְאֶחָד עֵרֵב בֵּין הַשְּׁמָשׁוֹת, שְׁנֵיהֶם קָנוּ עֵרוּב; שֶׁלְּאוֹתוֹ שֶׁנֶּאֱכַל עֵרוּבוֹ בֵּין הַשְּׁמָשׁוֹת אָנוּ חוֹשְׁבִים אוֹתוֹ לַיְלָה, וּלְאוֹתוֹ שֶׁהִנִּיחַ עֵרוּבוֹ בֵּין הַשְּׁמָשׁוֹת חוֹשְׁבִים אוֹתוֹ יוֹם. אֲבָל אִם עֵרֵב עָלָיו בֵּין הַשְּׁמָשׁוֹת וְנֶאֱכַל עֵרוּבוֹ בֵּין הַשְּׁמָשׁוֹת, אָסוּר.
3 If one made an eruv for two people—for one person while it was still day and that eruv was consumed during twilight, and for another person during twilight—both have acquired their eruv. For the one whose eruv was consumed during twilight, we consider it night; for the one whose eruv was placed during twilight, we consider it day. However, if one made an eruv during twilight and it was consumed during twilight, it is forbidden.