After the Beis Hamikdash was destroyed, our Sages, of blessed memory, ordained that on every joyous occasion, there should be a remembrance of the destruction of the Beis Hamikdash, as it is written: "If I forget you, Jerusalem, if I fail to elevate Jerusalem above my foremost joy" (Psalms 137:5). They decreed that no Jew should build a house¹ or Synagogue without any restrictions. (Biyur Halachah 560:1) for himself that is lime-plastered and decorated like a royal palace.² rules that if a house was merely painted, but not plastered, even though it was decorated, leaving an amah unpainted is sufficient. (see Mishnah Berurah 560:1) Neither should he lime plaster his entire house, but he may plaster the house,³ unplastered. (Ibid. 560:2) with clay and then lime plaster, it (lit. whitewash it), and he should leave a space of one square amah⁴ opposite the entrance⁵ above the door, because the main objective is that the inhabitants of the house see it and be reminded of the destruction, rather than those (strangers who enter the house on occasion). Leaving it opposite the entrance, only serves as a reminder, upon entering the house, but not as a reminder for those who spend time in the house. (Ibid. 560:3) unplastered,⁶ of bare walls. (Ibid. 560:3) as a reminder of the destruction [of the Beis Hamikdash. ] We do not know of any good reason why people do not observe this custom nowadays.

footnotes: ¹ It is, however, permitted to build a Beis Midrash ² Tur ³ If, however, the house was plastered with mud or clay, but not painted, it is not required to leave an amah ⁴ See glossary. ⁵ Others have the custom to leave the amah ⁶ It is not proper to have it painted black. Those who have curtains (wall-paper) around all the walls of the house, should also leave an amah

They also ordained that if you are setting a table, and are having guests for a meal, even for a meal that is considered a mitzvah, you should not serve all the dishes that are conventionally served at a meal.⁷. 560:5,6) They also ordained that a woman should not wear all her jewelry at one occasion,⁸ (560:2) questions why this custom and the previous custom are not observed at all (in our time and place.) and that a bridegroom before the chuppah [wedding ceremony], should have ashes put on his head on the place where the tefillin are worn, and the veil, with which the bride is covered, should not contain any silver or gold threads. It is also customary, that after the tenaim [engagement agreement] has been read to break a plate, as a reminder of the destruction of the Beis Hamikdash, but a damaged plate is used (for that purpose). Under the chuppah the bridegroom breaks a glass, and for this purpose, a whole glass may be used.

footnotes: ⁷ Omitting even a minor item is sufficient. On Shabbos or Yom Tov, however, nothing should be omitted from the traditional meals. (Ibid ⁸ Biyur Halachah

They also decreed, that we should not listen to musical instruments not even to the singing of songs. During meals, you should sing only the traditional zemiros [songs of praise], like those we sing on Shabbos; but it is forbidden to sing other songs.

Take heed not to attend animal shows presented by non-Jews⁹ 224:3) or their dances or any of their social functions. When you hear them giving voice to their happiness, you should sigh and agonize over the destruction of Jerusalem, and pray for it unto the Holy One blessed is He. Even animal shows presented by Jews, are forbidden because it is a "Gathering of scorners." All kinds of rejoicing are forbidden, except making a groom and a bride happy which may be done both by singing and with musical instruments. But even at a wedding, you should not overdo the merry making. It is forbidden for anybody to laugh exuberantly in this world, even when rejoicing in the performance of a mitzvah,¹⁰ 560:20) for it is said: "Then our mouth will be filled with laughter" (Psalms 126:2). [When Hashem will return the captivity to Zion].

footnotes: ⁹ This also applies to theatrical presentations, presented by Jews. (see Mishnah Berurah ¹⁰ Such as at a wedding or on Purim. (Ibid.