אורח חיים תע״וOrach Chayim 476

Se’if 1א׳

מָקוֹם שֶׁנָּהֲגוּ לֶאֱכֹל צָלִי בְּלֵילֵי פְּסָחִים, אוֹכְלִים. מָקוֹם שֶׁנָּהֲגוּ שֶׁלֹּא לֶאֱכֹל, אֵין אוֹכְלִין גְּזֵרָה שֶׁמָּא יֹאמְרוּ: בְּשַׂר פֶּסַח הוּא. וּבְכָל מָקוֹם אָסוּר לֶאֱכֹל שֶׂה צְלִי כֻּלּוֹ כְּאֶחָד בְּלַיְלָה זֶה, מִפְּנֵי שֶׁנִּרְאֶה כְּאוֹכֵל קָדָשִׁים בַּחוּץ; וְאִם הָיָה מְחֻתָּךְ אוֹ שֶׁחִסֵר מִמֶּנּוּ אֵבֶר אוֹ שָׁלַק בּוֹ אֵבֶר וְהוּא מְחֻבָּר, הֲרֵי זֶה מֻתָּר בְּמָקוֹם שֶׁנָּהֲגוּ. הַגָּה: וְלֹא יֹאכַל וְלֹא יִשְׁתֶּה הַרְבֵּה יוֹתֵר מִדַּאי, שֶׁלֹּא יֹאכַל הָאֲפִיקוֹמָן עַל אֲכִילָה גַּסָּה אוֹ יִשְׁתַּכֵּר וְיִישַׁן מִיָּד (מַהֲרִי"ל).

The custom of eating a roast on the night of Passover. Containing 2 paragraps: In a place where they are accustomed to eat a roast on Passover nights, one may eat it. In a place where they are accustomed to avoid eating it, one may not eat it, as a precaution against people saying 'this is the Passover sacrifice.' But in any place, it is prohibited to eat an entire roasted lamb as one on this night, because it appears like one eating sacrificial food outside the Temple. But if it was in parts, or had one limb missing, or if one limb was boiled even while still attached, it is permissible in a place where they are accustomed to it. Rema: One should neither eat more than sufficiently nor drink more than appropriately, so that one should not eat the afikoman with coarse eating or even to get drunk and sleep immediately.

Se’if 2ב׳

אֲפִלּוּ בְּשַׂר עֵגֶל וְעוֹף, כָּל דָּבָר שֶׁטָּעוּן שְׁחִיטָה, אָסוּר לֶאֱכֹל צָלִי בְּמָקוֹם שֶׁנָּהֲגוּ שֶׁלֹּא לֶאֱכֹל צָלִי. הַגָּה: נוֹהֲגִים בִּקְצָת מְקוֹמוֹת לֶאֱכֹל בַּסְּעֻדָּה בֵּיצִים, זֵכֶר לַאֲבֵלוּת, וְנִרְאֶה לִי הַטַּעַם מִשּׁוּם שֶׁלֵּיל תִּשְׁעָה בְּאָב נִקְבַּע בְּלֵיל פֶּסַח, וְעוֹד זֵכֶר לַחֻרְבָּן שֶׁהָיוּ מַקְרִיבִין קָרְבַּן פֶּסַח. וְיֵשׁ נוֹהֲגִין שֶׁלֹּא לֶאֱכֹל שׁוּם טִבּוּל בַּלַּיְלָה, רַק ב' טִבּוּלִים שֶׁעוֹשִׂים בַּסֵּדֶר (מַהֲרִי"ל).

Even beef or poultry, anything that requires slaughter, is prohibited to eat roasted in a place where they are not accustomed to eat a roast. Rema: They are accustomed in a few places to eat eggs at the meal, in commemoration of mourning, and it seems to me that the reason is because the (day of the week of the) night of Tisha B'av is set based on the night of Passover. Additionally, it is in commemoration of the destruction of the Temple, where they would bring the Passover sacrifice. And some are accustomed not to eat any dips on that night except for the two dips that we perform in the Seder (Mahari"l).