What if someone is “sitting Shiva” and hears that a friend or a relative has passed on? Can he go and join the funeral procession or visit the house of mourning?
According to the Gemara, during the first three days such an avel (mourner) cannot leave his home, but afterwards he can go to the house where others are “sitting Shiva“; there he should sit with the people in mourning rather than with the comforters. Rabbeinu Yehonatan and the Ran understand that this limits the mourner to attending the funeral (and home) of relatives, which explains why he joins those who are “sitting Shiva.” Most rishonim, however, do not distinguish between relatives and others.
Tosafot point out that this is the source for another tradition – that we allow mourners to come to the synagogue on Tisha b’Av. Since on the Ninth of Av all Jews are in mourning, as they indicate publicly by their behavior on that day (sitting on the floor, removing their leather shoes, etc.), the avel can take his place among them on this day.
Another difference between the first three days of aveilut and the rest of the week is the exchange of greetings. At the beginning of the mourning period, no greetings are permitted at all. In the latter part of the week, a mourner can respond to a greeting, but he cannot initiate one. Once the week is over, he can speak to people as usual.
The issue of greetings is taken very seriously by the Gemara. In Massekhet Berakhot the Gemara emphasizes the importance of returning greetings and tells stories about the greatest of Sages who made it their business to greet every person, no matter what their social standing or status. This sensitivity is what encourages the Sages to permit returning a greeting even at a time when greetings are, for the most part, forbidden. Even during the first three days of mourning, when the avel cannot respond to others, he should not simply ignore the greeting, but rather should explain that in his present situation as a mourner he cannot follow the normal behaviors of polite interaction.