Is Tachanun recited if there is a Chasan in Shul who is getting married that day?
It is customary not to recite Tachanun in the presence of a Chasan on the day that he enters the Chupah. Tachanun is omitted for the entire day, during all that day’s prayers [Shacharis and Mincha], on the day he enters the Chupah. [Some Poskim however rule that the above only applies if the Chasan is getting married during the day. If however the Chasan is getting married at night then some Poskim say that Tachanun is to be recited even by Mincha, even if he is present in Shul, while other Poskim say Tachanun is to be recited by Shacharis and only by Mincha is it to be omitted in his presence. Furthermore, some Poskim rule that even when the wedding is taking place during the day, if it is taking place in the afternoon Tachanun is omitted by Mincha and is recited by Shacharis. According to this opinion, Tachanun is only omitted by Shacharis when the Chuppah takes place before midday. However other Poskim rule that even when the Chuppah is taking place at night Tachanun is omitted in both Shacharis and Mincha of that day in the presence of the Chasan. Practically, the widespread custom is that if the Chuppah is taking place during the day, Tachanun is not recited even by Shacharis. If the Chupah is taking place at night many communities are accustomed to recite Tachanun by Shacharis in such a case. The widespread custom in many Chabad communities is not to recite Tachanun even in Shacharis, even when the Chuppah is taking place at night. Each community is to follow the ruling of their Rav.]
Days prior to day of Chupah: Tachanun is recited as usual in the presence of the Chasan on all days prior to the day he enters the Chupah.
Avoiding Davening in Shul: It is not necessary for the Chasan to avoid coming to Shul in order to allow the congregation to recite Tachanun, although some communities are accustomed to do so starting one to tywo days prior to the wedding.
Shacharis: Tachanun is omitted by Shacharis when Davening in the presence of the Chasan, if he is getting married during the day. If he is getting married at night, it is disputed as to whether Tachanun is to be omitted by Shacharis in the Chasan’s presence, and practically the widespread custom in many Chabad communities is not to recite Tachanun in Shacharis even when the Chuppah is taking place at night. Each community is to follow the ruling of their Rav.
Mincha: Tachanun is omitted by Mincha when Davening in the presence of the Chasan, even if he is getting married at night.
Is Tachanun to be recited during Mincha at the wedding hall?
No. Tachanun is not recited even if the Chasan is not present in the Minyan.
 131/5-6; Michaber and Rama 131/4
 Admur 131/5; Michaber ibid
This applies both in the house of the Chasan [when Davenining with the Chasan in his house-Admur 131/6] and when the Chasan is in the Shul. [Admur 131/5 and 6; Michaber ibid; Shivlei Haleket 30]
 Admur 131/5; Rama 131/4
The reason: As this day is his personal Yom Tov. [Admur ibid; See Mordechai Megillah 808; Rav M.S. Ashkenazi in Teshurah Bakodesh Penima]
Other Poskim: Some Poskim rule that Tachanun is only omitted once he gets married. [M”B 131/21 in name of Derech Hachaim 38/9; Chazon Ish] Other Poskim rule that if it is taking place in the afternoon, Tachanun is omitted by Mincha and is recited by Shacharis. [Birkeiy Yosef ibid; Ketzos Hashulchan 24/6]
 Admur 131/6; Rama ibid; Hagahos Maimanis 5; Divrei Nechemia 131 Hashlama
This is in contrast to the law by a Bris Mila in which Tachanun is only omitted during the prayer directly prior to the Mila, and hence if the Mila is taking place after Shacharis, Tachanun is omitted by Shacharis but recited by Mincha. [Admur 131/6; Rama ibid]
 See Poskim brought in next two footnotes; Nitei Gavriel Nissuin 6/4; In truth the case discussed in the Rama and Admur ibid is referring to a Chupah taking place during the actual day. [See M”A 131/12 in name of Darkei Moshe and Terumos Hadeshen 80 that this Halacha is referring to a Friday wedding, of which the Chuppah must take place during the day; Rav M.S. Ashkenazi in Teshurah Bakodesh Penima; Rav S.D. Levin in kuntrus Tanaaim p.118]
 Simple implication of Peri Chadash 131/4, brought in Beir Heiytiv 131/14; Radbaz 2/175 brought in Shaareiy Teshuvah 131/14; M”B 131/21 in name of Derech Hachaim 38/9; However see Birkeiy Yosef 131/5 that possibly even in their opinion Tachanun is to be omitted by the prayer prior to the wedding.
The reason: As so long as he has not yet gotten married he is not called a Chasan. [Peri Chadash ibid; M”B ibid in name of Derech Hachaim 38/9]
 Birkeiy Yosef 131/5 [brought in Shaareiy Teshuvah 131/14] that so is the custom of Eretz Yisrael, and it is a compromise between the opinions; Shalmei Tzibur p. 154; Zechor Leavraham 1/6; Chesed Lealafim 131/14; Yifei Laleiv 131/12; Kaf Hachaim 131/70; Ketzos Hashulchan 24/6 rules like this opinion; see Birkeiy Yosef 131/5 that possibly this applies even according to the Peri Chadash and Radbaz ibid.
 The reason: As Tachanun is to be omitted from the prayer which is prayed prior to the Chuppah. [ibid]
 Birkeiy Yosef ibid that so is custom of Eretz Yisrael; Ketzos Hashulchan 24/6; Yabia Omer 3/11-12
How the above Poskim argue on Rama/Admur: Although the Rama and Admur ibid explicitly rule that Tachanun is omitted the entire day, the above Poskim perhaps learn that the Rama and Admur refer only to once the Chasan is married that day, as explained in M”B ibid in name of Derech Hachaim 38/9, however see Nesiv Hachaim 38/2 which writes that According to Admur one does not say Tachanun that entire day, unlike the explanation of the Derech Hachaim ibid.] To note that Admur in the Siddur completely omits the day of the Chuppah as a day that Tachanun is not recited. Perhaps this is also a reason that the Ketzos Hashulchan does not rule as is the simple understanding of Admur in the Shulchan Aruch.
 Mahriy 80, brought in Peri Chadash 131/4 and in Beir Heiytiv 131/14; M”B 131/21 in name of Shulchan Shlomo and Gr”a; Kneses Hagedola, brought in Kaf Hachaim 131/70, writes that he wanted to nullify this custom until he found it written in Mahariy ibid; Some learn the Levush to also rule this way. [brought Shaareiy Teshuvah 131/14]
 The reason: Some suggest that the reason for this is because we are accustomed to write the Kesuba during the day and hence the Simcha has already begun. [Rav M.S. Ashkenazi in Teshurah Bakodesh Penima] Others suggest the reason is because the preparation for the wedding and its Simcha already begins 1-2 days before the Chuppahg, and hence one may be lenient at least regarding the day pf the Chuppah.
 Simple understanding of Admur and Rama ibid; Nesiv Hachaim 38/2; Aruch Hashulchan 131/16; Piskeiy Teshuvos 131/21
 Sheveit Halevi 7/18; Sheilas Shaul 19; Piskeiy Teshuvos 131/21
 Heard from the following Chabad Rabbanim: Rav M.S. Ashkenazi in Teshurah Bakodesh Penima; Rav Asher Lemel Hakohen; Rav S.D. Levin [see kuntrus Tanaaim p.118]; Rav Raskin in Siddur footnote 414 in name of Rav Z.S. Dworkin.
Ruling of other Chabad Rabbanim: The following Chabad Rabbanim are stringent that Tachanun is to be recited in Shacharis in the event that the Chuppah is taking place at night: Ketzos Hashulchan 24/6; Rav Ginzberg of Omer.
 Admur 131/6; M”A 131/12; Terumos Hadeshen 80; Rama ibid “He is only called a Chasan on the day he enters the Chupah”
Custom not to come to Some places are accustomed that the Chasan does not come to Shul for a day or two before his Chupah in order so the congregation can recite Tachanun, However in the event that the Chasan entered the Shul Tachanun is recited with exception to the day of the Chupah. [Admur ibid; M”A ibid; Terumos Hadeshen ibid] This implies that there is room to omit Tachanun even two days before the wedding. [see Rav Levin in kuntrus Tanaaim p.118]
 Admur 131/6; M”A 131/12
 Piskeiy Teshuvos 131/21
 The reason: As this has the same status as the hosue of the Chasan in which Tachanun is omitted., however see Admur 131/6 “when Davening with the Chasan”