Gemara

(Mishnah): We do not marry women during the Mo'ed, whether out of they are Besulos (virgins), or do Yibum, for it is Simchah for him;

However, one may remarry Gerushaso (his ex-wife).

(Gemara) Question: Why is 'it is Simchah for him' a reason to forbid marriage?

Answer #1 (Rav Yehudah or R. Elazar): We do not mix Simchos (the Mo'ed and marriage).

Answer #2 (Rabah bar Rav Huna): One might neglect Simchah of the Regel and engage (solely) in Simchah of his wife.

Abaye: This is like Rav, who expounds "V'Somachta b'Chagecha" - not in your wife.

Answer #3 (Ula): One will do excessive exertion on the Mo'ed for the wedding.

Answer #4 (R. Yitzchak Nafcha): People will (delay marriage until the Mo'ed, in order to fulfill Seudas Yom Tov through the wedding feast, and) defer Peru u'Rvu (the Mitzvah to have children).

Question (against all four answers - Beraisa): Marriage is forbidden during the Mo'ed, but one may marry the day before (even though the celebration lasts for seven days. All of the above concerns apply!)

Answers: The primary Simchah and exertion is only on the first day. People will not delay marriage until Erev Yom Tov, because only one day is allowed.

Question: What is the source that we do not mix Simchos?

Answer #1: On Erev Yom Kipur, Shlomo made a celebration "Shiv'as Yomim v'Shiv'as Yomim". The first week celebrated finishing the Beis ha'Mikdash; the second week was Sukos. If we may mix Simchos, he would have made one celebration during Sukos!

Rejection: Perhaps one may not delay in order to mix Simchos, but one may mix Simchos if the proper time is during the Mo'ed!

Answer #2: It says "Shiv'as Yomim v'Shiv'as Yomim Arba'ah Asar Yom" - this is extra to teach that we may not mix Simchos.

Kesuvos 47a: Surely, a man receives the earnings of his daughter (until Bagrus). If not, how did the Torah authorize him to send her to Chupah, and interrupt her work!

Objection (Rav Achai): Perhaps he compensates her for the lost work. Alternatively, he sends her to Chupah at night or on Shabbos or Yom Tov!

Rishonim

The Rif and Rosh (1:16) bring the Mishnah, all four reasons why we do not marry women during the Mo'ed, and the Heter to marry on Erev Yom Tov.

Nimukei Yosef (DH Masnisin and DH Gemara): Only a wife who is new to him is Simchah. If one mixes Simchos, he is not properly happy with either. We permit to marry on Erev Yom Tov. One will not delay marriage, since if he would not be able to marry that day he may not marry during the Mo'ed.

Rambam (Hilchos Yom Tov 7:16): We do not marry women during the Mo'ed, lest Simchas ha'Chag be forgotten amidst Simchas Nisu'in. However, one may remarry Gerushaso.

Beis Yosef (DH Kosav ha'Rambam): It seems that the Rambam forbids (Nisu'in during the Mo'ed, and also) Seudas Nisu'in during the Mo'ed, i.e. even if the Nisu'in was before the Mo'ed. However, perhaps this is not so. Rather, he forbids Seudas Nisu'in for one who remarried Gerushaso. Rashi, Hagahos Ashri and the Rivash all permit Seudas Nisu'in during the Mo'ed if the Nisu'in was before the Mo'ed.

Rambam (Hilchos Ishus 10:14): We do not marry women during the Mo'ed because we do not mix Simchos - "Malei Shevu'a Zos v'Nitnah Lecha Gam Es Zos".

Rosh (1:16): The Yerushalmi says that one may remarry Gerushaso from Nisu'in, but not if she was divorced from Kidushin, for then marrying her is a Simchah.

Tosfos (8b DH b'Chagecha): The Drashah ("b'Chagecha" - not in your wife) is not a mere Asmachta. The Gemara (Chagigah 8b) says that since Rav Ashi does not need it to teach about Korbanos, he uses it to forbid marrying during the Mo'ed.

Tosfos (Kesuvos 47a DH d'Mosar): "B'Chagecha..." is a Drashah mid'Oraisa forbidding marriage (even) on Chol ha'Mo'ed. How can we say (here) that a father could marry off his daughter on Yom Tov, when she may not work anyway? Perhaps it means just before Yom Tov. Tosefes Yom Tov, to cease working before Yom Tov starts, is mid'Oraisa. Alternatively, the father could hand her over to Sheluchei ha'Ba'al, or make Nisu'in without a Seudah. This is not such a big Simchah to be forbidden during the Mo'ed.

Question (Chachmas Shlomo): Tana d'Vei Shmuel's Beraisa permits Kidushin, but not Nisu'in or Seudas Eirusin. (This implies that Nisu'in is forbidden even without a Seudah!)

Answer #1 (Chachmas Shlomo): Tosfos permits Nisu'in without a Seudah if the Kidushin was before Yom Tov. The Beraisa forbids doing both Kidushin and Nisu'in during the Mo'ed. However, all the Poskim forbid even Nisu'in alone.

Answer #2 (Maharsha): Tosfos explains that mid'Oraisa, only Nisu'in with a Seudah is forbidden. Therefore, even if the Torah did not give a girl's earnings to her father, he could marry her off during the Mo'ed. Tosfos agrees that mid'Rabanan it is forbidden even without a Seudah.

Magen Avraham (OC 546:4): Mid'Rabanan one may not get married during Tosefes Yom Tov because this is a Kinyan, unless he did not accept Kedushas Yom Tov.

Answer #3 (Magen Avraham OC 546:1): Tosfos answers for Rav, who learns from "V'Somachta b'Chagecha" that the Seudah for the Chag may not be for your Kalah. Therefore, Nisu'in without a Seudah is permitted. We hold like Shmuel, who forbids mixing Simchos, and Nisu'in without a Seudah is a Simchah.

Poskim

Shulchan Aruch (OC 546:1): We do not marry women during the Mo'ed, whether they are Besulos or widows, and we do not do Yibum.

Mishnah Berurah (1): Even Nisu'in without a Seudah is a forbidden.

Shulchan Aruch (2): One may remarry Gerushaso from Nisu'in.

Beis Yosef (DH u'Mah she'Chosav Min): The Yerushalmi forbids remarrying Gerushaso from Kidushin. I do not know why the Rif and Rambam omitted this.

Mishnah Berurah (7): The Beis Yosef connotes that one may not make a Seudah for remarrying Gerushaso; some Acharonim are lenient. Surely, one may be lenient if it is not the day of the Nisu'in.

Shulchan Aruch (3): One may marry anyone the day before the Regel and make a Seudah during the Mo'ed, whether she is a Besulah or widow.

Question: If Nisu'in was before the Regel but not a Seudah, may one make Seudas Nisu'in during the Mo'ed?

Answer (Beis Yosef DH veha'Rashba): In a Teshuvah, the Rashba says that the primary Simchah is the Nisu'in itself. As long as it was before the Mo'ed, Seudos Nisu'in during the Mo'ed are permitted, even though they are Mitzvos that exempt Shushbinin from Sukah. In another Teshuvah he forbids Seudos Nisu'in during the Mo'ed even if the Nisu'in was beforehand. However, if Panim Chodoshos came, Simchas Mo'ed does not override the Mitzvah to make a Seudah and bless Sheva Berachos. In a third Teshuvah he says that the Gemara permits Seudos Nisu'in during the Mo'ed if the Nisu'in was beforehand. We may be lenient, since also Rashi permits and it is a mid'Rabanan matter.

Rebuttal (Magen Avraham 4): Tosfos says that the Drashah is mid'Oraisa!

Kaf ha'Chayim (12): Some are lenient to make Nisu'in on Erev Yom Tov close to dark. This is improper. The Gemara permitted Erev Yom Tov because the primary Simchah is one day. Therefore, the Nisu'in should be in the morning, to enable the Chasan to rejoice with the Kalah a day before Yom Tov. Also, a Seudah on Erev Yom Tov should begin before Minchah. In pressed circumstances, the Nisu'in may even be close to dark.

Mishnah Berurah (10): (We are lenient to allow Seudas Nisu'in if it is not the day of the Nisu'in.)`We permit the first Seudah to be at night, for it is considered a new day. However, the Magen Avraham says that regarding this it is considered the same day, therefore the custom is not to have Nisu'in on Erev Yom Tov. In pressed circumstances one may be lenient.