Gemara

(Beraisa): "V'Hayisa Ach Some'ach" is extra. It includes [in the Mitzvah of Simchah] the last night of Yom Tov.

(Beraisa) Question: Why do we include the last night and exclude the first night?

Answer: Simchah applies before the last night, but not before the first night.

Mo'ed Katan 8b (Mishnah): We do not marry women during the Mo'ed, for it is Simchah for him.

Question: Why is this a reason not to marry?

Answer #1 (Rav Yehudah): We do not mix one Simchah with another.

Answer #2 (Rabah bar Rav Huna): It is lest one abandon Simchah of the Regel and engage [only] in Simchah of his wife;

(Rav): We do not marry women during the Mo'ed, for it says "v'Samachta b'Chagecha", and not [to rejoice] in your wife.

Answer #3 (Ula): It is due to toil.

Answer #4 (R. Yitzchak Nafcha): It is due to neglect of Peru u'Rvu. (People will delay marriage until the Regel, to fulfill Seudas Mo'ed through Seudas Nisu'in.)

Question (against all of these opinions - Beraisa): Whoever may not marry during the Mo'ed, is permitted Erev ha'Regel.

Answer (for all the opinions): The primary Simchah and toil is only one day. People will not delay marriage just for one day.

Sukah 25b (Mishnah): A Chasan and Shushbinav (his close friends) and all the Bnei ha'Chupah are exempt from Sukah all seven days, for they need to rejoice.

Question: They could eat in the Sukah, and rejoice in the Chupah!

Answer: Simchah is only in the place of the Seudah.

Kesuvos 47a - Question: If a father does not receive the earnings of his Na'arah, how did the Torah authorize him to send her to Chupah, and interrupt her work?

Answer (Rav Achai): Perhaps he sends her to Chupah at night or on Shabbos or Yom Tov (when she does not work)!

Rishonim

Rambam (Hilchos Yom Tov 7:16): We do not marry women or do Yibum during the Mo'ed, lest one forget Simchas ha'Chag due to Simchas Nisu'in. One may remarry his divorcee or be Mekadesh during the Mo'ed, as long as he does not do Seudas Erusin or Seudas Nisu'in, lest he mix another Simchah with Simchas ha'Chag.

Hagahos Ashri (Sukah 2:8 DH v'Chen): One may have Nisu'in on Erev ha'Regel. It is a Mitzvah to rejoice all seven days. The primary Simchah is one day.

Teshuvas ha'Rashba (7:9): If one was married on Erev Yom Tov, he may not make Seudas Nisu'in during the Mo'ed, for it is Simchah for him. However, if Panim Chodoshos came by themselves, perhaps we bless Sheva Berachos, for a Chasan requires Berachah whenever there are Panim Chodoshos. Perhaps we are not Mevatel the Berachah due to Simchas ha'Mo'ed.

Rivash (260): Chachamim permitted Nisu'in on Erev ha'Regel. Even though he will rejoice with his wife the entire Regel, the Isur is only on the day of Nisu'in, for it is the primary Simchah.

Rashi (8b DH l'Chad): Erev ha'Regel is the only day for the Nisu'in that will "save" a meal. One would not delay until then, lest he be unable to make the Nisu'in that day.

Tosfos (8b DH Lefi): A verse about Shlomo (he finished the Mikdash before Yom Kipur) teaches that we do not mix one Simchah with another. The Yerushalmi learns from "Malei Shevu'a Zos." This resembles the Isur to do Mitzvos in bundles. One's heart should be open [to intend fully] for one Mitzvah. He should not veer from it. Likewise, one's heart should be open for [one] Simchah

Tosfos (Chagigah 8b DH b'Chagecha): "V'Samachta b'Chagecha" [and not in your wife] is a proper Drashah. It is not a mere Asmachta, for in Chagigah (8b) we ask what Rav Ashi expounds from it.

Tosfos (Kesuvos 47a DH d'Masar): "V'Samachta b'Chagecha" forbids marrying during Chol ha'Mo'ed, for we do not mix one Simchah with another. This is a proper Torah Drashah (Chagigah 8b). I answer that the father sends her to Chupah just before Yom Tov. Tosefes Yom Tov is mid'Oraisa, so in any case she cannot work then. Alternatively, he sends her with the husband's Sheluchim, or does Nisu'in without a Seudah. Even though there is Simchah in Nisu'in without a Seudah (Yevamos 43b), it is not enough to forbid this on Yom Tov.

Poskim

Shulchan Aruch (OC 546:3): Anyone may marry on Erev ha'Regel and make the Seudah during the Regel, whether a virgin or widow.

Beis Yosef (DH u'Mah she'Chasav uv'Erev): The Rambam connotes that even if one married before the Mo'ed, he may not make Seudas Nisu'in during the Mo'ed. However, one could explain that this is only when he married during the Mo'ed, e.g. he remarried his divorcee, but one who married on Erev ha'Regel may make Seudas Nisu'in during the Mo'ed. Rashi connotes like this. He permits l'Chatchilah Nisu'in on Erev ha'Regel, even though the seven days of Seudah will be during the Regel. Also Hagahos Ashri and the Rivash connote like this.

Beis Yosef (DH veha'Rashba): The Rashba was asked about the Heter to marry on Erev ha'Regel. Perhaps he must eat the first meal on Erev ha'Regel, for the Simchah is only where the meal is. He answered that the primary Simchah is the Nisu'in. It may not be during the Mo'ed. The meal is secondary to it. Even though there is Simchah all seven days of Seudos Nisu'in, and it is enough of a Mitzvah to exempt the Chasan and his close friends from Sukah, [it is permitted during the Mo'ed]. In another Teshuvah (7:9), he forbids making Seudas Nisu'in during the Mo'ed, but if Panim Chodoshos came by themselves, perhaps we bless. In another Teshuvah he permits Nisu'in on Erev ha'Regel and Seudos Nisu'in during the Regel. The Gemara answered why there is no problem of delaying Peru u'Rvu, toil, or mixing Simchos. Since this is a mid'Rabanan matter, and he himself permits [in two Teshuvos - PF], like Rashi, we are lenient. One may invite people to Seudas Nisu'in during the Mo'ed, since the Nisu'in was Erev ha'Regel. This is the custom.

Magen Avraham (4): In one Teshuvah the Rashba forbids making Seudas Nisu'in during the Regel. Also the Rambam connotes like this. The Beis Yosef is lenient, since it is mid'Rabanan. This is astounding. Tosfos in Chagigah and Kesuvos proved that the verse is a proper Torah Drashah. Sukah 25b connotes that one who married on Erev ha'Regel may make a Seudah during the Regel. I forbid making a Seudah the first night, for the Gemara says that the primary Simchah is one day. The night follows (gets the law of) the [preceding] day. The Simchah begins during the day and finishes at night. According to the opinion that it is due to toil, there is excessive toil. One may make a Seudah during the rest of the Regel, for the primary Simchas Nisu'in already passed. Tosfos (Kesuvos 47a) permits marrying shortly before Yom Tov. This is mid'Oraisa, but mid'Rabanan one may not marry then, for this is like acquisition (339:4). However, if he did not yet accept Yom Tov, it is permitted. The Rit (this is not the Ritva, for he permits) in Kesuvos forbids marrying close to dark. Keneses ha'Gedolah says that the custom is to marry even close to dark. Here the custom is to marry on Erev Shabbos close to dark, but not to marry on Erev Yom Tov at all. I say that it is due to the Isur to make Seudas Nisu'in the first night. Therefore, one should not change the custom.

Mishnah Berurah (9): The Acharonim say to be careful to make the Nisu'in in the morning, so he can rejoice with her one day before the Regel, and so the Seudah will be on a Yom Chol. If he was delayed, and it is pressed circumstances, Eliyahu Rabah permits Nisu'in even close to dark. The Magen Avraham says that the custom is not to marry at all on Erev Yom Tov. If there is a great need, one should not be stringent.

Mishnah Berurah (10): It seems that the Shulchan Aruch permits the Seudah even at night, for the primary Simchah is the start of the Nisu'in, and the night is a new day. The Magen Avraham forbids. He holds that the night follows the day, and this is why we do not marry on Erev ha'Regel. In pressed circumstances, one may be lenient even to make the first meal at night.

Sha'ar ha'Tziyun (15): The Magen Avraham holds that the Isur of Nisu'in during the Mo'ed is mid'Oraisa, therefore he forbids the first meal at night, for there is concern for a Torah Isur. Eliyahu Rabah and Bigdei Yesha rejected this, for all agree that a Seudah without Nisu'in is only mid'Rabanan. Chemed Moshe rejected the Magen Avraham for a different reason. The Mitzvas Aseh of Simchah [of the Regel] is not on the first night, like it says in Pesachim. Also Sha'agas Aryeh (68) concludes like this. If so, the Beis Yosef is correct. One should not be stringent about this.

Kaf ha'Chayim (12): If a Nisu'in is on Erev Yom Tov, l'Chatchilah it and the Seudah should be in the morning, for on the afternoon of Erev Yom Tov one may not eat a [big] meal. B'Di'eved, one may marry until close to dark, and make the Seudah at night.