... בְּתִשְׁעָה בְאָב נִגְזַר עַל אֲבוֹתֵינוּ שֶׁלֹּא יִכָּנְסוּ לָאָרֶץ, וְחָרַב הַבַּיִת בָּרִאשׁוֹנָה וּבַשְּׁנִיָּה, וְנִלְכְּדָה בֵיתָר, וְנֶחְרְשָׁה הָעִיר. מִשֶּׁנִּכְנַס אָב, מְמַעֲטִין בְּשִׂמְחָה:
...on the 9th of Av, it was decreed that our ancestors should not enter the Holy Land; on the same day the first and second Temples were destroyed, the city of Bethar was taken, and the site of [Jerusalem] was ploughed. From the 1st of Av, one is bound to lessen joy [until after the fast on the 9th].
דאז מימי המרגלים נועד החודש הזה לצרת ישראל:
Commenting on the statement of the Mishnah, "From the 1st of Av one is bound to lessen joy" - Since the days of the Spies (Meraglim, whose evil report of the Land of Israel is recorded in the book of Bemidbar/Numbers in Parashat Shelach), this month has been associated with calamity for the Jewish People.
The following selections from Kitzur Shulchan Aruch introduce the customary restrictions of the Nine Days. There are many details and special circumstances that govern our observance of these customs. These sources are shared here for educational purposes, not for establishing halachic practice. If you have specific questions, you are encouraged to consult your Rav.
(ז) מִשֶּׁנִּכְנַס אָב, מְמַעֲטִין בְּשִׂמְחָה. אֵין בּוֹנִין בִּנְיָן שֶׁל שִׂמְחָה אוֹ בִּנְיָן שֶׁהוּא רַק לִרְוָחָה. ...
(7) When the month of Av arrives, one is bound to reduce joy. One should not build a building meant for enjoyment or strictly for relaxation. ...
(ח) מִנְהָג בְּכָל יִשְֹרָאֵל שֶׁלֹּא לֶאֱכֹל בָּשָׂר וְשֶׁלֹּא לִשְׁתּוֹת יַיִן בְּתִשְׁעַת יָמִים שֶׁמִן רֹאשׁ חֹדֶשׁ עַד לְאַחַר תִּשְׁעָה בְּאָב, ...וּבִסְעוּדַּת מִצְוָה, כְּגוֹן מִילָה וּפִדְיוֹן הַבֵּן, וְסִיּוּם מַסֶּכְתָּא גַּם כֵּן, מֻתָּרִין בְּבָשָׁר וְיָיִן. ...
(8) It is a custom throughout the Jewish world not to eat meat or drink wine during the nine days, from Rosh Chodesh until after Tishah beAv. ... At a meal that is considered a mitzvah, such as a brit (circumcision), a pidyon haben (redemption of the first-born son), or the conclusion of a Talmud tractate, it is permitted to eat meat and drink wine. ...
(ט) וְכֵן אֵין מְכַבְּסִין בְּתִשְׁעָה יָמִים אֵלּוּ. וַאֲפִלּוּ חָלוּק אוֹ בֶּגֶד שֶׁאֵינוֹ רוֹצֶה לְלָבְשׁוֹ עַד אַחַר הַתַּעֲנִית. .... וְכֵן אָסוּר בְּתִשְׁעָה יָמִים אֵלוּ לִלְבּוֹשׁ אוֹ לְהַצִּיעַ אֲפִלּוּ הַמְכֻבָּסִין מִקֹּדֶם. רַק לִכְבוֹד שַׁבָּת, מֻתָּר לִלְבּוֹשׁ בִּגְדֵי פִּשְׁתָּן, וּלְהַצִּיעַ עַל הַשֻּׁלְחָנוֹת לְבָנִים, וּלְהַחֲלִיף מִטְפְּחוֹת הַיָּדַיִם וּמַגָּבוֹת, כְּדֶרֶךְ שֶׁעוֹשִׂין בִּשְׁאָר שַׁבָּתוֹת....
(9) One should not wash any laundry during the nine days; not even a shirt or a garment that one would not wear until after the fast day. ...During these nine days, it is also forbidden to put on laundered clothing, or to spread linen, even if they were washed before. However in honor of Shabbat, one may put on clean linen garments (undershirts and socks) and spread a fresh tablecloth on the table, and change washcloths and towels, just as is done for other Shabbatot.
(יב) אֵין רוֹחֲצִין בְּתִשְׁעָה יָמִים אֵלּוּ אֲפִלּוּ בְּצוֹנֵן. אַךְ לִרְפוּאָה, כְּגוֹן יוֹלֶדֶת אוֹ מְעֻבֶּרֶת שֶׁקְּרוֹבָה לָלֶדֶת, שֶׁטּוֹב לָהּ לִרְחוֹץ, וְכֵן אָדָם חָלוּשׁ שֶׁצִּוָּה אוֹתוֹ הָרוֹפֵא לִרְחוֹץ, מֻתָּרִין לִרְחוֹץ אֲפִלּוּ בְּחַמִּין.
(12) One should not bathe during these nine days, even in cold water. [Note: The Kitzur Shulchan Aruch does not address bathing for the sake cleanliness, which is a mark of contemporary life.] However, as relates to healing, as, for example, when a woman has given birth, or a pregnant woman, close to childbirth, for whom bathing is beneficial, or a weak person, whose physician ordered him to bathe, these are permitted to bathe, even in warm water. ...