Gemara

(Mishnah): If Reuven was away when his relative died, if he was nearby, and returned within the first three days of Avelus, he counts the (seven and 30) days of Avelus like the mourners here;

If he comes after three days, or came from afar, he counts the days of Avelus from when he heard of the death;

R. Shimon says, if he was close, even if he arrives on the seventh day, he counts like the mourners here.

The Halachah follows R. Shimon;

(Shmuel): In Aveilus, the Halachah always follows the lenient opinion.

Mo'ed Katan 21b (R. Yochanan): If he comes within three days from close, he counts like them. This is only if the oldest of the household was in the house.

Question: If the oldest went to the cemetery, what is the law?

Answer: R. Yochanan taught that even if the oldest went to the cemetery, he counts like them.

Question (Beraisa): He counts by himself.

Answer: R. Yochanan discusses when he came within three days. The Beraisa discusses when he came after three days.

R. Shimon's law is only if he came and found consolers.

Question (Rav Anan): If the consolers stirred to stand (and leave), but did not stand, what is the law?

This question is not resolved.

(R. Yochanan): The Halachah follows R. Shimon.

Rishonim

Rif (Mo'ed Katan 13b): The Gemara explicitly rules like R. Shimon. This is if he found people consoling the oldest.

Rif and Rosh (Mo'ed Katan 3:38): The Ge'onim explained that 'a close place' is within 10 Parsa'os, which is as far as one goes in one day. Since had he heard he would have come within one day, it is as if he was here.

Rambam (Hilchos Evel 7:4): If one's relative died, and he did not know until he came, if he was close, i.e. within 10 Parsa'os, which one can traverse in one day, even if he arrives on the seventh day, if he found consolers with the oldest of the household, even if they stirred to stand (and leave), since he found consolers, it counts (like Shivah) for him, and he counts with them the rest of Sheloshim. If he did not find consolers, he counts by himself. Similarly, if he was far away, even if he came on the second day, he counts by himself Shiv'ah and Sheloshim from the day he came.

Rosh (ibid.): The Ra'avad explains the Shi'ur as follows. If he died close to night and Stimas ha'Golel (sealing the coffin cover) was just before dark, and the next day they sent a Shali'ach to inform Reuven, and he arrived towards the end of the day, Reuven can come and arrive within three days, which is still the intense part of Aveilus. If he is further, perhaps he could not come within three days. If he came from further, even if he came within three days, he counts by himself. If he came after three days, even if he came from close, he counts by himself. To count with them he must come within three days from a place where it was apt he will come within three days if they need to send a Shali'ach (to inform him). R. Shimon requires that he come from a place where he was apt to arrive within three days, but he does not require that he arrive within three days.

Rosh: The Ra'avad gave a mere Siman that he is close enough so that he could find out through a Shali'ach and come within three days. Really, we require that he did not know of the death until he came to his brothers. If he knew on the second day and became mourning, it is absurd to shorten his Aveilus just because he came to his brothers. However, if he did not begin at all and came to his brothers, it is as if he began Aveilus with them. The Gemara asked about when the oldest went to the cemetery for the needs of the Mes, e.g. to make a monument or needs of the structure of the grave. R. Yochanan rules like R. Shimon, who says that it suffices to come within seven days. Here he distinguished between when Reuven came within or after three days! This is when the oldest went to the cemetery. I saw other opinions in Halachos of Maharam, but I disagree.

Poskim

Shulchan Aruch (YD 375:8): If one's relative died, and he did not know until he came to the place where he died, or to the place of burial, if he was close, i.e. within 10 Parsa'os, from where one can come in one day, even if he arrives on the seventh day, if he found consolers with the oldest of the household, even if they stirred to stand, since he found consolers (i.e. they were still conducting some Aveilus), it counts for him, and he counts with them the rest of Sheloshim. If he did not find consolers he counts by himself. Similarly, if he was far away, even if he came on the second day, he counts by himself Shiv'ah and Sheloshim from the day he came.

Beis Yosef (DH veha'Ramban): The Ramban says in the name of Bahag that if he comes within three days, even from afar, he counts with them. If he comes after three days from afar, he counts by himself. If he came from nearby, he counts with them, like R. Shimon. The Ritz Gei'us questioned this. We never said that he counts with them when he came from afar! R. Shimon argues only about one who came from nearby. If he came from afar, all agree that he counts by himself. Since R. Shimon discusses only one who came from nearby, how do we establish R. Yochanan's law that it depends on whether he came within three days? That is like the first Tana.

Beis Yosef (DH v'Rabbeinu): R. Chananel says that if he comes from nearby within three days, he counts with them. This is if the oldest is there, or he went to the cemetery and returned within three days. If Reuven came after three days and found that the oldest went to the cemetery, he counts by himself. If he came from afar within three days, he counts by himself. After this (three days), even if he came from nearby, he counts by himself. R. Shimon says that if even if he comes on day seven from nearby, he counts with them. R. Yitzchak says that this is more precise. It seems that they agree that if he comes from nearby he counts with them if the oldest is there, even if he came on day seven, like R. Shimon. If the oldest was not there at all, and his Aveilus did not take effect with theirs, Reuven does not count with them, whether he came on day two or day seven. If the oldest went to the cemetery, and Reuven came from nearby, if the oldest returns within three days, Reuven counts with them. It is as if he found him in the house. If the oldest delayed in the cemetery until after three days, Reuven counts by himself, even if he came from nearby on day two, and all the more so if he came after three days and the oldest was still in the cemetery. This is because the oldest himself when he returns within three days counts with them, even if he was far away in the cemetery, since he was initially with them and returned to them. Therefore, Reuven counts with them and with the oldest. All of them have one count. If the oldest returned after three days from the cemetery, he counts by himself, even if the cemetery was close, therefore also Reuven does not become like the younger mourners. Also Tosfos says like this. Tosfos says that it is not only the oldest, rather, even if the youngest went to the cemetery.

Beis Yosef (DH veha'Rif): The Rif omitted the law of when the oldest went to the cemetery. The Sugya connotes that it is according to the first Tana. Others asked that R. Yochanan taught this, and he rules like R. Shimon! I hold like the Rif, that R. Shimon is not concerned for days. The third day is like the seventh. It does not matter if the oldest or youngest went. R. Yochanan merely resolves the Beraisa, and establishes it like Chachamim. R. Yochanan holds like R. Shimon regarding days. The oldest must be in the house. If he came from elsewhere, he is not drawn after the younger mourners. When the oldest went to the cemetery, the Gemara asked whether he is drawn after them, or they are drawn after him, and count only from Stimas ha'Golel. R. Yochanan answered that he counts with them. We challenged him from the Beraisa, and he answered that it discusses one who comes after three days, like Chachamim. The oldest coming from the cemetery is like a younger mourner coming from elsewhere. This is whether the cemetery is close or far. Surely the oldest begins Aveilus only from Stimas ha'Golel, but when he joins the mourners he counts with them. This is the best Perush of the Sugya.

Beis Yosef (DH veha'Mordechai): Since the Rif and Rambam agree, we rule like them.

Shach (11): If Reuven came to a place other than the place of death or burial, everyone counts by himself.

Rema: Some say that even if one came, and the oldest was not in the house, rather, he went for the needs of the Mes, if he returned within three days, it is as if he is in the house, and one who comes within three days counts with them. One can be lenient like this opinion. Whenever the law is that he counts with them, even if he returned afterwards to his house, he counts with them.

Beis Yosef (DH Kasav, citing R. Yerucham): Tosfos says that the law is the same whether the oldest or even the youngest went to the cemetery. If Reuven comes within three days, he counts with them. If he comes after three days, he counts by himself, for he rules like R. Yochanan.

Shach (12): The Darchei Moshe said that the Rosh holds like Tosfos and R. Chananel, so we hold like them, for in Aveilus, the Halachah always follows the lenient opinion. Also, they are the majority, and the Rosh is Basra.

Shach (12): When a woman dies, her husband is considered the oldest, for he is obligated to bury her more than her brother or father. It does not depend on who is most pained, for people have different mindsets. The Rashba says so.

Gra (20): The Rif, Rambam and Shulchan Aruch omitted the law of when the oldest went to the cemetery, for they hold that it is like the first Tana. The Rosh and others brought it, for they hold that R. Shimon agrees in this case.

R. Akiva Eiger: Bedek ha'Bayis (DH v'Chasav Od) brings from the Rashba (532) that if the oldest came to Reuven on day two and informed him, Reuven does not count with him. One who comes is drawn after the resident, but the resident is not drawn after the one who comes.

R. Akiva Eiger (DH Ho'il): If he comes on Shabbos (when we do not console), we can say that he must come before sunrise.