Gemara

(Reish Lakish): Hash-m arranges matches so that a man always gets a woman corresponding to his deeds - "a Tzadik is not subjected to authority of a Rasha".

(Rabah bar bar Chanah): Matchmaking is as difficult as splitting the Yam Suf - "Elokim Moshiv Yechidim Baysah Motzi Asirim b'Kosharos."

Question: Shmuel taught that 40 days before a fetus is formed, a Bas Kol (voice from Heaven) announces whose daughter is for him to marry!

Answer: Shmuel discusses one's initial Zivug (match). Above we discuss a second Zivug.

Mo'ed Katan 18b (Shmuel): One may be Mekadesh a woman even during Chol ha'Mo'ed, lest another be Mekadesh her first.

Question: Shmuel taught that every day, a Bas Kol announces whose daughter is for Ploni!

Answer: Perhaps another will come first through (prayer for) mercy.

Rabah heard a man pray that he marry Plonis.

Rabah: This is improper. If she is destined for you, you will not lose her. If she is destined for you, you deny Hash-m (you will think that prayer does not help)!

Afterwards, Rabah heard the man pray 'he (I) or Plonis should die (before she marries someone else, lest I be pained).' He rebuked him, for Hash-m decrees Zivugim - "me'Hashem Yotza ha'Davar"...

Rishonim

The Rif and Rosh (Mo'ed Katan 10b and 3:23) bring Shmuel's teaching.

Nimukei Yosef (DH b'Rachamim): Even though it was decreed whose daughter will marry Ploni, perhaps Almoni, who is not her Zivug, will pray and Hash-m will listen. The decree will be fulfilled; Almoni will divorce her or die early, and Ploni will marry her afterwards.

(Bach OC 551 DH v'Hachi): The concern is lest Almoni pray that he (himself) or she die, lest he be pained by seeing another marry her. We are not concerned lest Almoni marry her, for the Bas Kol said whom she will marry! If she is not destined for Almoni, his prayer will not change this!

Tashbetz (2:1): Zivugim are decided before the fetus is formed. Children, lifespan and income do not depend on merit, rather, on Mazel (Mo'ed Katan 28a). It is a Mitzvah to marry, so Hash-m does not decree whether or not a person will marry. However, if he will choose to marry it is decreed which girl he will marry. Prayer and merit can change the Zivug, therefore one may be Mekadesh a woman even on Tish'ah b'Av. Ohev Mishpat says that the Gemara concludes that everything depends on merit. Even the opinion that Yesh Mazal l'Yisrael says so only regarding children, lifespan and income.

Question (Tosfos Mo'ed Katan 18b DH u'Mi): The Bas Kol is only for one's first Zivug (Sanhedrin 22a, Sotah 2a). We should have answered that Shmuel's concern applies to one's second Zivug!

Answer (Bach, ibid.): The Gemara answered why it is permitted even if he and she were not married before.

Rambam (Hilchos Ishus 10:14): One may be Mekadesh a woman on any weekday, even on Tish'ah b'Av.

Magid Mishneh: The Yerushalmi permits, lest another be Mekadesh her first.

Hagahos Maimoniyos (30): R. Tam (Sefer ha'Yashar 48:10) said that with great difficulty, Chachamim permitted to be Mekadesh on Shabbos. He was unwilling to rule in practice.

Rambam (Hilchos Evel 6:5): An Avel may not make Nesu'in for 30 days. He may be Mekadesh a woman even on the day his relative died.

Rosh (48): A Beraisa forbids Nisu'in during the 30 days of Aveilus. The same applies to Kidushin, like the Mishnah in Yevamos (41a). The Beraisa discusses Nisu'in to teach that it is permitted after 30 days.

Kesef Mishneh (Evel 6:5): The Rambam permits Eirusin during Aveilus, just like on Tish'ah b'Av. The Rosh (and Ramban) say that we are lenient only about an old Aveilus (over the Churban). The Rambam does not distinguish, for the concern is lest someone else merit her through mercy.

Rambam (Shemoneh Perakim, Perek 8 DH Aval ha'Lashon): Getting married is either an Aveirah (if she is forbidden to him) or a Mitzvah (Peru u'Revu, if she is permitted), therefore one cannot say that Hash-m decrees who will marry whom. Anything pertaining to human free choice is not b'Yedei Shomayim.

Rashi (Sotah 2a DH Eini): Reish Lakish said that Zivgum are according to one's deeds. This is unlike Shmuel, who says that Zivugim are determined before birth, before it is determined whether he will be a Tzadik or Rasha, for Yir'as Shomayim is in a person's hands. We answer that Reish Lakish refers to Zivug Sheni.

Rebuttal (Yad ha'Melech Hilchos Teshuvah 5:5): Even though a person has choice and Hash-m knows what he will choose, Rashi explains that decrees about Zivugim are not based on His knowledge. However, in Sanhedrin (22a) we challenged Rabah bar bar Chanah (who says that Zivugim are as hard as Kri'as Yam Suf) from Shmuel. If Zivugim were announced beforehand, they would be no harder than arranging other matters. Reish Lakish was not mentioned there. Surely, also in Sotah the question was only against Rabah!

Poskim

Shulchan Aruch (OC 551:2): From Rosh Chodesh Av until Tish'ah b'Av, we do not marry or have Seudas Eirusin. One may be Mekadesh without a Seudah, even on Tish'ah b'Av itself, lest someone else be Mekadesh her first.

Question (Taz 3): The Tur says that Eirusin without a Seudah is not Simchah. If so, why did the Gemara need to say that we are concerned lest another be Mekadesh her first? It seems that he holds that this concern does not help for Aveilus. The Tur forbids an Avel to be Mekadesh.

Question (Beis Yosef 524 DH Aval): The Tur says that R. Tam permits one without a wife or children to be Mekadesh on Yom Tov, but Rashi forbids in every case. Why doesn't he say that they argue also about Yibum?

Answer (Taz 1): R. Tam is lenient only about Kidushin, lest another be Mekadesh her.

Levush: Once one is Mekadesh her, others despair and will not pray.

Shulchan Aruch (EH 61:2): It is proper to have Nisu'in only when she is Tehorah.

Rema: Nowadays we are not particular about this; we do not wait. In any case, it is good to inform the Chasan that she is Nidah.

Beis Yosef (YD Sof Siman 192 citing Hagahos Maimoniyos): SMaK permits Chupas Nidah when the Chasan is informed, because Yisrael are few in number; perhaps someone else will marry her first. R. Yechiel is stringent, for perhaps Chivas Bi'ah acquires, and this does not apply to a Nidah.

Shulchan Aruch (392:1): An Avel may be Mekadesh a woman even on the day his relative died.

Pischei Teshuvah (4): In our lands, we are Mekadesh just before Nisu'in. The Heter to be Mekadesh within three festivals includes Nisu'in, for if not, she will not agree to Kidushin, and perhaps someone else will Mekadesh her first. We do not extend this leniency to permit Nisu'in during Chol ha'Mo'ed or the nine days, or in places where Kidushin is a fair time before Nisu'in.

Chasam Sofer (YD Sof Siman 350): Chachamim permitted Eirusin during Chol ha'Mo'ed even for Zivug Rishon, even though it is remote that another's prayer will help. Surely we may be lenient about Zivug Sheni during Aveilus!