Mitzvah tantz (lit. "mitzvah dance" in this case the word "mitzva" means "commandment" in both Hebrew and Yiddish, and the word "tantz" means "dance" in Yiddish) is the Hasidic custom of the men dancing before the bride on the wedding night, after the wedding feast. Therefore the word "mitzvah" here denotes "custom" because it is not something commanded in the Torah. Commonly, the bride, who usually stands perfectly still at one end of the room, will hold one end of a long sash or a gartel while the one dancing before her holds the other end. There are times when one of the leading rabbis, usually her father or grandfather, will dance with her as well.
Background to the custom
The custom evidently predates Hasidism, being mentioned first in the medieval Machzor Vitri, and has its basis in the Talmud (Ketubot), where there is an expression ketsad merakdim lifnei hakallah "how does one dance before the bride?" Although some Orthodox groups oppose this practice, most Hasidim have maintained this ancient custom and consider a great honor to be able to dance in front of the bride to give her honor on her wedding night.
The Talmudic background to this custom can be seen from the following exchange:
- Kesuvos 016: "Dancing" before the Kalah
- Why does the gemorah start the sugya with Kaitzad *merakdim* lifnei ha Kalah? ("How does one dance in front of the bride?") The Gemorah then talk about talking then singing then *dancing*? Dancing is last. Even more the 3 things of talking singing & dancing are really ways of giving simcha ("joy"). So the sugya should start Kaitzad MESAMICHIM Es ha Kalah ("how does one give joy to the bride?") and not Kaitzad merakdim lifnei ha Kalah ("how does one dance before the bride?")
- The words "Keitzad Merakdin" ("how does one dance?") are not part of the Gemara. They are part of a Beraisa which the Gemara cites. The Beraisa *only* discusses what to say before the Kalah, and makes no mention of dancing at all. The Maharsha, though, says that "Kalah Na'eh va'Chasudah" ("pleasant and righteous/beautiful bride") is a song, and not just a statement, so it is not just referring to "talking" as you mentioned in your question. Nevertheless, your question remains; it should have said "What do we sing" or "Keitzad Mesamchin." (Chochmas Mano'ach has a few approaches that deal with this, but they are along the lines of Drush ("interpretation").)
- The answer might be that if one walks up to a Kalah and tells her, "You are so beautiful and charming!", it is immodest. First of all, the person must look at the Kalah when addressing her to her face - and the Gemara 17a frowns upon such behavior. Second, the person is directing everyone else's attention to examine the beauty of the Kalah.
- The solution, then, is to dance before the Kalah and include references to the beauty of the Kalah as part of the song and dance. Since the focus of his actions is the dance, he is not necessarily looking at the Kalah (it would be polite even if he is looking at his feet). In addition, the person is not directing the audiences attention to the Kalah's beauty as much as to the dance.
- Of course the Kalah, who is the subject of the praise, revels in the praise despite it being just "part of the dance."
In addition there is some discussion about the application of the custom:
- The g'mara obviously takes for granted that dancing in the kallah's presence is done, and asks how it is done. However, the "how" refers not to the feet, but to the mouth -- what is sung to accompany the dancing. After all, on the Mishna in Beitzah 36b, Rashi comments that m'rakdin (dancing) (along with m'tapchin and m'sapkin) is "l'simcha ul'shir ("for joy and song")
- How does 'riqud' ("dancing") take on the meaning of 'amirah' ("talk")?" (see RaShY ad loc.), no need to worry: from the sugya on 17a, esp. "amru alav al R' Y'hudah bar Ila-ai shehayah notail bad shel haddas umraqaid lifnai hakkallah v'omair, 'Kallah na'ah vachasudah!'" (they said about Rabbi Yehuda bar Illai that he would take a stalk of myrtle and dance before the bride and say: "pleasant and righteous/beautiful bride") it would seem that "Kaitzad m'raqdin" ("how does one dance?") is answered as if the Q was "Kaitzad om'rim" ("saying") only because the Q can't possibly pertain to the details of the dancing (really, as per T'hilim 114..)
Bride and groom rejoice
During the mitzvah tantz, the bride and a few women, usually her relatives and some important rebbetzins, are brought to be in the men's section and there is no mechitza separating them. In some instances the mechitza will be moved aside entirely with all the women present seated facing the men on the other side. If there is a large crowd at a wedding of a notable rabbi, most of the women will be looking down from a higher women's gallery.
The groom and most of his male relatives take turns rejoicing in front of the new bride at the time of the mitzvah tantz. In the case of the marriage of children or grandchildren of notable rebbes, it becomes an opportunity for the entire community, followers and admirers of the rabbis' involved to watch and rejoice as the mitzvah tantz is done by the leading rebbes and rabbis in attendance. This may go on all night until dawn.
The custom of honoring a bride is related to the notion of the Jewish Godhead having both male and female components that seek unification to be united as one, as God's "feminine nature" is called the Shechina in the feminine, and a marriage between a Jewish man and woman is regarded as the symbol of a yichud ("unification") of God's "oneness" with the joining of a bride and groom. By rejoicing in front of a bride and creating an atmosphere of happiness and joy at that moment in the wedding it is, as it were, as if one were rejoicing before the Shechina and the moment of its highest unification with utmost Godliness, which is a reason for great rejoicing. In addition, a Jewish groom and bride are considered to be as if they were a king and queen on their marriage day, so by honoring them through dance one is honoring the "queen" and her "king."
- Satmar Rebbe of Kiryas Joel, NY dancing mitzvah tantz with his grandson's bride
- The Bobover Rebbe dancing mitzvah tantz
- The late Kretchnifer Rebbe dancing mitzvah tantz
- Nadvorna Rebbe, from Bnei Brak, dancing mitzvah tantz
- Sanz Klausenberg Rebbe dancing mitzvah tantz
- Biala Rebbe of Boro Park dancing mitzvah tantz
- Deejsher Rebbe of Boro Park dancing mitzvah tantz with his daughteryi:מצווה טאנץ