סימן קיט סעיף א
אַף עַל פִּי שֶׁבְּכָל שַׁבָּת וְיוֹם טוֹב יְכוֹלִין לְקַדֵּשׁ וְלֶאֱכֹל מִבְּעוֹד יוֹם לְהוֹסִיף מֵחֹל עַל הַקֹּדֶשׁ, בַּפֶסַח אֵינוֹ כֵן, לְפִי שֶׁמִצְוַת אֲכִילַת מַצָּה הִיא דַּוְקָא בַּלַּיְלָה, כְּמוֹ קָרְבַּן פֶּסַח, דִּכְתִיב בֵּיה וְאָכְלוּ אֶת הַבָּשָׂר בַּלַּיְלָה הַזֶּה. וְכֵן מִצְוַת אַרְבַּע כּוֹסוֹת הִיא דַּוְקָא בַּלָּיְלָה. וְכֵיוָן שֶׁגַּם הַכּוֹס שֶׁל קִדּוּשׁ הוּא אֶחָד מֵאַרְבַּעַת הַכּוֹסוֹת, לָכֵן אֵין מְקַדְּשִׁין עַד שֶׁהוּא וַדַּאי לָיְלָה. יִלְבַּשׁ אֶת הַקִּיטֶל (חָלוּק לָבָן) וְיִתְיַשֵׁב עַל מושָׁבוֹ לַעֲשׂוֹת אֶת הַסֵּדֶר. וּמִצְוָה לְחַלֵּק לַתִּינוֹקוֹת שְׁקֵדִים וֶאֱגוֹזִים וְכַדּוֹמֶה, כְּדֵי שֶׁיִּרְאוּ שִׁנּוּי וְיִשְׁאֲלוּ, וְעַל יְדֵי זֶה יִתְעוֹרְרוּ לִשְׁאוֹל גַּם כֵּן עַל מַצָּה וּמָרוֹר וַהֲסִבָּה. וְתִינוֹק וְתִינֹקֶת שֶׁהִגִּיעוּ לְחִנּוּךְ, דְּהַיְנוּ שֶׁהֵם יוֹדֵעִים בִּקְּדֻשַּׁת יוֹם טוֹב, וּמְבִינִים מַה שֶּׁמְסַפְּרִים מִיצִיאַת מִצְרַיִם, נוֹתְנִים לָהֶם גַּם כֵּן כּוֹס שֶׁיִּשְׁתּוּ מִמֶּנּוּ. נוֹהֲגִין לִמְזוֹג כּוֹס אֶחָד מִן הַמְסֻבִּין, וְקוֹרִין אוֹתוֹ כּוֹס שֶׁל אֵלִיָּהוּ הַנָּבִיא (תע"ב ת"פ).
Although on every other Shabbos and Yom Tov, you are permitted to say kiddush and eat a meal while it is still day, in order to add from the secular to the holy, on Pesach, you are not permitted to do so. The mitzvah of eating matzah must be performed only at night, as was the case with the Pesach sacrifice, about which it is written, "And they shall eat the meat (of the Korban Pesach) on this night." (Exodus 12:8). The mitzvah of the Four Cups may also be done only after nightfall. Since the cup of wine, over which we say kiddush, is one of the Four Cups, the kiddush should not be said until it is definitely night.1It is required to wait, therefore, until after the appearance of the stars. See glossary. (The person who conducts the seder) puts on the kittel and takes his seat to conduct the seder. It is a mitzvah to distribute almonds, nuts and similar things to the children, so that they notice the change and ask questions, and this will also stimulate them to ask also the reasons for eating matzah, maror and reclining.2These questions are included in the text of the Haggadah, Mah Nishtanah. It is important to keep the children at the table until after Avodim Hayinu is recited and explained to them, because this section of the Haggadah explains the questions, previously asked by the children. (Mishnah Berurah 472:50) Boys and girls who have reached the age of training for mitzvos, that is, who are able to understand the holiness of Yom Tov and understand what is being told about the Exodus from Egypt, should also be given a cup of wine,3It is considered a mitzvah to give them a cup of wine, but it is not required to do so, since many Poskim ruled that the mitzvah of the Four Cups was never ordained for children. (Ibid. 472:46,47) from which they should drink. It is customary to fill an additional cup of wine,4It is not clearly stated, when the Cup of Elijah should be filled. The fact that the Kitzur Shulchan Aruch mentions it, in this paragraph, could indicate that he maintains that it be filled, when the first cups are filled for Kiddush. Mishnah Berurah mentions the filling of the Cup of Elijah, in the paragraph dealing with the text, שֽׁפוֹךְ חֲמָתְךָ (Pour out Your wrath etc.), which comes much later after Birkas Hamazon. (Ibid. 481:10) and it is called "The Cup of Elijah the Prophet."
תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)