Can a mourner within the year of his father's passing attend the wedding of a nephew or a very good friend?

A mourner should not participate in a wedding feast within twelve months of his father's passing. He may attend the ceremony after thirty days if it is outside the hall and without music. If, due to the joy of the groom or bride, his presence is required during the meal, he may serve the guests as a waiter and eat with them, but he should leave during the dancing.

When the presence at the wedding of the groom or bride is necessary, one may attend the wedding of a nephew or a good friend and eat, as stated in the responsa of Maharshal, part Yoreh De'ah, sign 212, regarding the participation of brothers within the year: "We find that brothers are permitted to enter the wedding of their brother within the year, as there are no closer relatives than they, and it would be distressing for the groom if they did not participate in his wedding. And this is all the more so, as the mitzvah of Sukkah, which is a Torah commandment, is set aside so as not to distress the groom, the mourning of twelve months, which is the lightest of the light, all the more so." And there it is explained that even according to the opinion of the Raavad, who is stringent and holds that only at the wedding of an orphan and orphaness is it permitted, this refers to the wedding of a non-Jew who is not part of the wedding party, but for brothers who are part of the wedding party, it is permitted. And there it is permitted even within thirty days, as stated in the responsa of Sheilat Yavetz (part 2, sign 179).

If there is no necessity, one may attend the ceremony outside the hall, as stated in the Tur, sign 391: "And Rabbi Yehuda Albarceloni wrote that there are those who enter within twelve months to the ceremony to hear the blessing or to visit, but not to eat, and there are those who are stringent not to enter at all, and Ramban wrote that it is forbidden to enter there at all, whether during the meal or during the entertainment of the groom and bride, and so wrote my father the Rosh, and so is the custom in Ashkenaz, that for all twelve months they stand outside the house to hear the blessings and do not enter the house at all." And so ruled the Shulchan Aruch to forbid, and the Rema wrote: "And all this in the house where they make the wedding and eat and drink and rejoice there, but at the ceremony made in the synagogue, where they bless the blessing of betrothal and marriage and there is no joy at all, it is permitted immediately after seven days (Hagahot Maimoni). And there are those who forbid until thirty (there in the name of Ravia), and so it seems to me."

At the meal, by serving, as the Rema wrote, sign 391: "There are those who permit a mourner to eat at a wedding meal or circumcision with the servers, provided it is not in a place of joy, such as in another house (Kol Bo and Beit Yosef in the name of Smak), and there are those who forbid (Hagahot Ashiri), and so is the custom, only the mourner serves there, if he wishes, and eats at home from what is sent to him from the meal." Thus, the opinion of the Rema is that even when serving at the meal, he eats at home, however, the Raak Eiger wrote: "And in Belz it is written that the custom in Poland is that the groomsmen also go to eat, only that they first serve a little." And the custom in Poland to permit a little serving at first is only for close relatives and groomsmen, but for neighbors, the custom is to be stringent. However, one may be at the meal as a true server from beginning to end, as the Rema wrote, and eat at home or outside.