Halakhah su Salmi 90:71

After completing the Shemoneh Esrei, half-Kaddish is said followed by "vihi no'am," ["may the pleasantness"] which is said because it is a Psalm of blessing⁷Psalms 90: 17, 91. with which Moses blessed Israel when they completed the construction of the Tabernacle. It should be recited while standing.⁸During the first verse (Mishnah Berurah 295: 1) and preferably for the entire chapter. [Shaarei Teshuva 295] It is customary to repeat the final verse: "Orech yamim etc." ["With long life"]. After that "Ve'atah kadosh" ["and You, Holy One"] and the kedushah-order are said, since it relates to "Vihi no'am," because through the construction of the Tabernacle, the Shechinah dwelled amongst Israel. This is [the interpretation of]: "And You, Holy One, are enthroned upon the praises of Israel."⁹Taz 295. If Yom Tov occurs during the following week, even if on Friday, "Vihi no'am" and "Ve'atah kadosh" are not said, since included in "Vihi no'am" is [the prayer:] "Umaasei yadeinu koneneihu" ["and the work of our hands—establish it"]. Therefore, it is required that all six weekdays be fit for work. And once "Vihi no'am" is not said then, "Ve'atah kadosh" is also not said, because they relate to one another. After [completing] "Vihi no'am" and "Ve'atah kadosh," whole- Kaddish is said followed by "Veyiten lecha" and "Aleinu." These prayers are said after Maariv on Shabbos night, so that the Bnei Yisroel can prolong the completion of the order [of prayers], in order to delay the return of the wicked to Gehinnom; because it waits for them until the last assembly in Israel completes its order [of prayers].¹⁰Until then, some of the holiness of Shabbos remains. [Gra] This is based on Zohar, Midrash Tanchumah—Ki Sissa, Pesikta Rabasi.

Synagogues and study halls have great holiness since we pray in them and learn in them Torah. These locations have a reflection of the sanctity of the Temple. Our wise ones of blessed memory said, "And these will be small Temples" (Megilah 29a)-these are the Synagogues and study halls. And this that is written, "O Lord, You have been our dwelling place throughout all generations"(Psalms 90:1)- these are the Synagogues and study halls. Even in the time of the destruction [of the Temple], they still have sanctity as it is written, "and I will desolate your sanctuaries" (Lev. 26:31)-"They are still sanctuaries even when they are desolate" (Megilah 28a). Therefore, Synagogues and study halls are not treated with lightheartedness, for example, with games, humor and small talk. The Arizal (R' Issac Luria) was very careful to only speak words of prayer in the Synagogue. Even words of rebuke should not be spoken, least it lead to the discussion of mundane matters. (Magen Avraham Subsection 3).

The order of the [9 Av] day: [Eliezer ben Yoel haLevi] wrote in the Avi haEzri: the night of 9 Av they take off their sandals and go to the synagogue and sit on the ground like mourners and light no more than one light, with which to read the Book of Lamentations and kinot. And the shaliaḥ tzibur stands and prays Aravit and says the full Kaddish and reads the Book of Lamentations and says kinot, and after that the Kedusha d’Sidra, and starts with “And You are holy." And "To Zion will come a redeemer" is not said, for there is no redemption that night, nor "And as for Me, this is My covenant," for it seems to be establishing a covenant over kinot, and there is no relation to say "And as for Me, this is My covenant" for all is exempted then. But in a mourner's house it is said, for if the mourner is exempted, the comforters are not. And he says Kaddish without "May they be accepted." And they go home, and do not greet one another except in the manner of mourners or chastened ones. And if 9 Av ends on Shabbat or the day after, they do not say "Your Justice is justice." Just as on a new moon that falls (being on the Sunday after) Shabbat, that they do not say "Your Justice is justice" on Shabbat that is 9 Av it is called a mo'ed. And they do not say Psalm 90:17 [and Psalm 91]. And some of the Gaonim wrote that they do not say Psalm 90:17 [and Psalm 91], and also not to say the Kedusha d'Sidra. And Rav Zemaḥ Gaon wrote they do not say Psalm 90:17 [and Psalm 91] but they say "And to Zion will come" and all of the Kedusha d'Sidra, except for "And as for Me, this is My covenant."And Rabbeinu Nissim wrote that they do not say Psalm 90:17 [and Psalm 91] but it is our custom to say in the evening and morning to say "And as for Me, this is My covenant." And why not say it? And the people studies Job and Jeremiah and the kinot in the morning and does the order of blessings and hymns according to other days and there are places where it is customary not to say the Song [of the Sea]. And those praying the eighteen blessings individually say "Answer us" within "Hear our Prayer," and the shaliaḥ tzibur between "Redeemer" and "Healer" as on other fast days, and says "Comfort" within "Builder of Jerusalem." And unlike other public fast days twenty-four blessings aren't said and a Closing of the Gates prayer is not said. And Rav Amram wrote it is our custom to increase seliḥot in "Forgive us." And teḥinot are not said, as it is called a mo'ed, and if it falls on Monday or Thursday one says "God, long-suffering" but does not say "And He is merciful" and in Spain they do not say "God, long-suffering." And they take out a Torah scroll and read three aliyot from the section of Va-etḥanan, "When you bear children." (Deuteronomy 4:25–40). And the maftir is the third. And the maftir reads from Jeremiah, "I will gather, gather them" (Jeremiah 8:13 - 9:23). And [Eliezer ben Yoel haLevi] wrote in the Avi haEzri that one rolls the Torah scroll in its place so as not to reduce its honor. But in Tractate Soferim 18:4 it is written: Some read the book of Lamentations in the evening, and some delay it until the morning to after the reading of the Torah scroll. After the reading of the Torah scroll, one stands and wallows in ash, and they puncture their clothes and read in weeping and wailing. If they know how to translate, all the better. And if not, give it to one who knows how to translate so that all the people and the women and children will understand, for women are required as are men, as are male children. And the one who reads on 9 Av says "Blessed is the True Judge." And some place the scroll case on the ground and say "Fallen is the crown of our head" (Lamentations 5:16) and tear their clothes and weep like a person whose dead lies before them. And some change their places, and some get down from their benches. And all wallow in ashes and do not greet one another all night and all day until all the people have finished their kinot. And at them time of kinot it is forbidden to talk or go outside, so as not to stop one's heart from grieving. And similarly, not to talk with idolaters. And if there is a mourner in the town, he goes at night to synagogue, and in the day, until they finish kinot. And if there is an infant to circumcise, they circumcise him after they finish kinot. And some delay the circumcision until after noon. And some say that we do not bless over a cup but bless without a cup. And according to the Tosafot we bless on a cup but give it to a child to drink. and we are not concerned that perhaps he may be drawn to drink from it even after he grows up, because it is not a permanent matter. And therefore, on 9 Av that falls on the day after Shabbat we don't say Havdalah and give it to a child, because we are concerned that he may be drawn to it, for it is considered a permanent matter, because, according to the structure of the years, it sometimes falls on Shabbat once every three or four years. A sandak changes into other clothes, but not really white ones. One time when 9 Av fell on Shabbat and was postponed to the next day, Rabbeinu Yaavets was a sandak, and he prayed Minḥa while the day was still going and washed and did not finish his fast since it was a festival for him. And evidence can be found from where it is taught (Eruvin 51a) "Said Rabbi Eliezer bar Tzadok: I am a descendant of Senaav ben Binyamin. One time, the Ninth of Av fell on Shabbat, and we postponed it until after Shabbat, and we fasted but did not complete it because that it was our Festival." And some have the custom not to slaughter or prepare necessities for the break-fast until after noon. At Minḥa they read "And Moses implored" (Exodus 32:11-14, Exodus 34:1-10) as on other fast days, and the haftarah is "Seek out" (Isaiah 55:6–56:8). And Rabbeinu Hai wrote that his custom was to read "Return" (Hosea 14:2-10) as the haftarah. And they pray the eighteen blessings and say "Comfort" within "Builder of Jerusalem" and "Answer us" within "Hear our Prayer," and the shaliaḥ tzibur between "Redeemer" and "Healer."