Are you allowed to get married the night going into Shiva Asar B'Tamuz? If you have the Chupa prior to Shekiya can the wedding continue past sunset? (Sources please)
Rabbi Moshe Feinstein says yes, "if needed". (I.e. there's a legitimate reason why you couldn't do the wedding a day or two earlier.)
His cousin-once-removed, Rabbi Joseph Dov Soloveichik, said no.
Rabbi Soloveichik's argument: "if the Talmud debated whether the fast starts at night, and some later rabbis thought you start saying the fast-day prayers at night, that means that elements of the fast really start at night."
Rabbi Feinstein's argument (OC I:168): "the Talmud concludes that the fast doesn't start at night; yes it was later debated about saying the prayers at night, but we conclude that the fast-day prayers don't start until day; that opinion has stronger Talmudic support; and the whole no-three-weeks-wedding thing is a custom, so we should be lenient in cases of doubt."
Hat tip Joseph Kaplan and others.
On the surface level, yes, but more importantly is whether one should. The three weeks are a rather auspicious time and over the centuries Jews have tried to prevent taking on major activities during that time. My recommendation would be to hold off until after 9th of Av, the 15th is especially an auspicious day to get married. There are also questions of sheva brachos during the three weeks, one can have them, but again, the mood is not ideal and has certain overtones. Its probably ideal to delay if possible, otherwise, its not asur.
From Halachically Speaking (Vol 6, Issue 8), "The Three Weeks":
There is a discussion among the poskim if one has to refrain from the actions which are not done during the three weeks from the morning of Shiva Asar B’tamuz or even the night before? Most poskim say that one should refrain from doing these activities even from the night before, from tzeis. In a pressing situation one can be lenient (i.e. wedding on Motzei Shabbos).11
From Footnote 11: "...This is even with music etc (See Igros Moshe E.H. 1:97)
The custom of Ashkenazim is to refrain from making weddings from the night of Shiva Asar B’tamuz until after Tisha B’av. This is even if one has not fulfilled the mitzvah of having children.
There was a big fight over this in Israel, I believe during the 70's / 80's. The chief rabbinate was dominated by Ashkenazim and they forbade weddings during the three weeks. One of the many things Rav. Ovadia Yosef is known for was his fight for Sephardi traditions, one of which is that weddings are allowed during the three weeks:
As a leading Sephardic Torah scholar and arbiter of halakha, Rabbi Yosef was often described by his followers as the greatest of the generation and the outstanding Sephardic rabbinical authority of the century. His most outstanding quality was his courage to proudly renew the Sephardic system of legislation. Time and again he disagreed, openly and proudly, with the official position of the rabbanut, while sticking to his religious Sephardic position. Thus, for example, when he was the Chief Rabbi of Tel Aviv, he cancelled the religious council’s decision not to permit marriage in the Ben Hametzarim (three weeks) days—as the Ashkenazim do—and instructed all the marriage registrars in his jurisdiction to allow marriage of Sephardic Jews for the whole month of Tamuz.