The custom of ashkenazic Jewry is not to get married during the three weeks between the seventeenth of Tamuz and Tisha B’Av. The sefardic custom is to permit weddings until Rosh Chodesh Av, but from Rosh Chodesh until after Tisha B’Av, it is prohibited. (Shulchan Aruch and Rama 501:1 and Yabia Omer 6:43)
Harav Ephraim Greenblatt zt”l (Rivevos Efraim 6:289) writes that it is prohibited for an ashkenazi to attend a wedding of a sefardic couple that takes place between the seventeenth of Tamuz and Rosh Chodesh Av. Contrastingly, Harav Moshe Shternbuch shlit”a (Teshuvos V’Hnahagos 4:128) disagrees and allows for ashenazim to attend sefardic weddings. A similar permissible view can also be found in the Sefer Netai Gavriel (Bein Hamitzarim 14:7), by Harav Gavriel Zinner shlit”a.
Harav Moshe Feinstein zt”l (Igros Moshe 1:168) writes that if necessary an ashkenazi may get married the night before the fast of the seventeenth of Tamuz. The prohibition against getting married would only begin on the morning of the fast.
However, Harav Shmuel Wosner zt”l disagrees and states that because he feels that the mourning period begins on the night of the seventeenth, therefore weddings are prohibited. The Tzitz Eliezer (10:26) also rules stringently. He wonders how Rav Moshe could omit the fact that many poskim, most notably the Chida, feel that the laws of the “Three Weeks” begin the night of the fast and therefore weddings should also be prohibited. He adds that one should not even get married during bein hashmashos of the night before the fast. Most authorities do not allow for weddings to take place the night of the seventeenth.
If the fast falls out on Shabbos and is delayed until Sunday (as was the case this year), one may not marry on Motzei Shabbos according to all authorities. Even though the fast is delayed until Sunday, the mourning period already begins on Shabbos. (Igros Moshe ibid.)