Law of a Grandson Whose Grandfather Passed Away

A person whose father is sitting shiva for the passing of his grandfather, what are his obligations in mourning?

If he is with his father who is sitting shiva, in the same city, he has 4 obligations: 1. He should not bathe in hot water (lukewarm is allowed). 2. He should not listen to music. 3. He should not wear Shabbat clothes on weekdays. 4. He should not attend joyous events such as weddings, Sheva Brachot, and other celebratory meals. However, he is allowed to attend mitzvah meals such as a brit, bar mitzvah, and siyum, but he is forbidden to eat unless the meal is at his home or he is the host of the celebration.

He is permitted to have a haircut and cut his nails, and he does not need to change his place in the synagogue.

In the Gemara Moed Katan, page 20, side 2: "Just as one mourns for them, so one mourns for their relatives, according to Rabbi Akiva. Rabbi Shimon ben Elazar says: one only mourns for one's grandson and one's grandfather. And the Sages say: anyone who mourns for him, mourns with him. The Sages are the same as the first Tanna! - The difference between them is regarding being with him in the house. As Rav said to his son Chiya, and so Rav Huna said to his son Raba: in his presence - observe mourning, in his absence - do not observe mourning. And so the Shulchan Aruch, Yoreh De'ah, Siman 374, paragraph 6, ruled that close relatives should observe mourning only in the presence of the mourning relative.

And the Rema wrote there: "And so it is customary nowadays not to mourn at all with the mourners, and anyone who is stringent in this is only doing so out of wonder. Nevertheless, it is customary that all close relatives of the deceased, who are disqualified from testimony, show some mourning themselves during the first week, that is, until after the first Shabbat, by not washing and not changing some of their clothes as on other Shabbats." In Masat Binyamin it is written that one should not change at all from weekday clothes to Shabbat clothes. In Shach, s"k 7, it is written that the prohibition of washing is only in hot water but is allowed in lukewarm.

And regarding celebratory meals: the Bach wrote: "All the more so, one should not enter a meal, even a mitzvah meal, until after Shabbat," and the commentators wrote that he means one may enter but not eat.

And regarding listening to music: the commentators wrote that logically it is forbidden, and although we do not forbid anything except what the commentators wrote, nevertheless, since joy and mourning are two opposites, they wrote logically to forbid music. And at a mitzvah meal, they wrote to permit listening to music.

And regarding cutting nails and hair: there are those who forbid, but the commentators ruled according to the opinion of Shevut Yaakov, who permitted it.

And regarding changing place in the synagogue: the Shach wrote that there is no need to change.