Should I go to a Baseball Game during the Three Weeks?

From when the month of Av begins, one decreases acts of rejoicing. During the week in which the Ninth of Av occurs, it is prohibited to cut one’s hair and to launder clothes, but if the Ninth of Av occurs on a Friday, on Thursday these actions are permitted in deference to Shabbat. On the eve of the Ninth of Av a person may not eat two cooked dishes in one meal. Furthermore, one may neither eat meat nor drink wine.

The Sages taught: All mitzvot practiced by a mourner [in aninut] are likewise practiced on the Ninth of Av: It is prohibited to engage in eating, and in drinking, and in smearing oil on one’s body, and in wearing shoes, and in conjugal relations. It is prohibited to read from the Torah, from the Prophets, and from the Writings, or to study from the Mishna, from the Gemara, and from midrash, and from collections of halakhot, and from collections of aggadot.

Simchat Me'reut- A meal that is made for friends and beloved ones that each does, even when it is not so joyus - [a mourner is permitted to go to this after 30 days ] - but this does not include a [very] joyful feast like a wedding.

(2) These are the practices forbidden to a mourner for the entire 30-day period. He is forbidden to cut his hair, to wear freshly ironed clothing, to marry, to enter a celebration of friends, and to go on a business trip to another city; five matters in all.

(7) When does the above apply? When one is mourning for other deceased persons. When mourning for one's father or mother, by contrast, under all circumstances, one is forbidden to enter a friendly gathering for twelve months.

...

(9) When mourning for all other deceased persons, if one desires, one may reduce his business activities. If he does not desire, he need not reduce them. When mourning for one's father or mother, by contrast, one should reduce one's business activities.

§ The mishna states that Rabbi Yosei says: All [previously married] may be betrothed within three months except for a widow, due to the mourning period she must observe for her deceased husband. Rav Ḥisda questioned the mishna’s ruling and said: Based on an a fortiori inference, it should be permitted for a woman to be betrothed during the thirty-day mourning period for her husband: If during another period of mourning, which the Gemara will specify, when it is prohibited to launder clothes, it is permitted to be betrothed, then during the thirty-day mourning period for a husband, when it is permitted to launder clothes, isn’t it logical that it should also be permitted to be betrothed? The Gemara explains the cases involved: What is the period of mourning to which Rav Ḥisda is referring? As we learned in a mishna (Ta’anit 26b): During the week in which the Ninth of Av occurs, it is prohibited to cut hair and to launder clothes, but on Thursday it is permitted in deference to Shabbat. And it is taught in a baraita: Prior to this time the public reduce their activities, refraining from business transactions, from building and planting, and they may betroth women but may not marry them, and they may not make a betrothal feast.

[ie, if mourning a spouse is like mourning over the destruction of the Temple, why must a widow must wait three months to get engaged, while someone could get engaged - but not married - at the beginning of the month of Av?]

Rather, it must be that the question of whether it is permitted to betroth a woman during the period prior to the week of the Ninth of Av is subject to a dispute between tanna’im. As it is taught in a baraita: From the New Moon of Av and until the fast on the ninth, the public reduces its activities, refraining from business transactions, from building and planting, and from betrothing and marrying. During the week in which the Ninth of Av occurs, it is prohibited to cut hair and to launder clothes, and it is prohibited to be betrothed. And some say: It is prohibited for the entire month. ... Rather, Rav Ashi stated a different resolution of Rav Yosei’s opinion: New mourning, i.e., the mourning for a relative who has just passed away, is different from old mourning, i.e., the mourning over historic events such as the destruction of the Temple, and it is appropriate to be more lenient in the latter. And the mourning of the public is different from the private mourning of the individual... As such, it is not possible to form an a fortiori inference from the halakhot of mourning for the Temple to those of a wife mourning for her husband.

"All her pursuers have overtaken her within the straits,” in days of distress, between the seventeenth of Tamuz and the ninth of Av, during which Ketev Meriri is prevalent, just as it says: “Nor the pestilence that stalks in darkness, nor the destruction [ketev] that lays waste at noon” (Psalms 91:6)...Rabbi Abahu...saw a certain person who was carrying a stick and going to strike another person. He saw a demon standing behind him carrying an iron rod. [Rabbi Abahu] stood and called out to him, saying to him: ‘Why do you seek to kill your friend!?’

[The man] said to him: ‘Can a person kill another with this?’ [Rabbi Abahu] said to him: ‘There is a demon standing behind him that is carrying an iron rod. You strike him with this and it will strike him with that and he will die.’

Rabbi Yoḥanan would instruct the teachers of Bible and the teachers of Mishna not to raise a strap to the children during those days.

Anonymous, (1200-1300*)
And I have seen dear women (nashim yikarot) who refrain from eating meat and drinking wine from the seventeenth of Tammuz until the tenth of Av and say that this is how they received it from their mothers generation after generation. And it seems to me that this because [as it says in the Gemara] on the seventeenth of Tammuz, the daily offering (the korban tamid) was cancelled, as well as in mourning, we do not drink wine. And some men also have this cusom.

From Rosh Chodesh until the Fast we minimize business transactions and building of joy, example the wedding house for his son or the building of molding and decorating (i.e. of walls and ceilings). Or joyous planting, example a royal banqueting canopy that is planted for shade to stand in its shade or types of myrtle and other tents. If its wall is leaning to fall even though it is of joy, it is permitted to build and for the needs of a mitzvah, all is allowed [Ran Last Chapter of Taanis]. We do not wed women and we not make engagement meals, but engaging without a meal is allowed. Even on Tisha B'av itself, it is permitted to engage that one not precede him. Note (Rema): Our custom is to be strict that we do not wed from the 17th of Tammuz and on until after Tisha B'av .

[EN: Note that the Kol Bo's observations have now become accepted Ashkenazi custom]

And what is the reason that it is permitted to make a match [but not to have a wedding]? At any rate it seems to me that it is assur to have a seudah, even if there is no dancing and no group-dancing ...and it seems to me that it is assur to dance or group-dance from the 17th of Tammuz and onwards.