A group of Sages – Rabbi Hiyya bar Abba, Rabbi Ami and Rabbi Yitzhak Nappaha – were sitting together and raised one of the most basic questions about the mourning period: how do we know that aveilut (mourning) is seven days long?
The answer presented by our Gemara is based on a passage in Amos (8:10): “…and I will turn your Festivals into mourning.” This is understood to teach that just as biblical holidays are seven days long, similarly the mourning period is seven days long. In response to the query that one holiday – Shavuot – is only celebrated for a single day, the Gemara quotes Reish Lakish who teaches that there is also a one-day mourning period. When a person hears of his loss thirty days or more after the person died, it is called a shemu’a rehoka (distant tidings) and he/she only “sits Shiva” for one day.
The Talmud Yerushalmi considers a wide variety of biblical passages that could be considered the source of the seven days of aveilut. Among them:
The waters of the flood engulfed the world after seven days (Bereshit 7:10), which is understood as God’s mourning period on the occasion of the destruction of the world.
Yosef declared a seven-day mourning period when his father, Ya’akov, passed away (Bereshit 50:10).
When Aharon ha-Kohen’s sons, Nadav and Avihu, were killed for bringing an improper sacrifice (Vayikra 10:1-2) their brothers remained in the Mishkan for the next seven days.
Miriam was struck with tzara’at, which Moshe likened to death (Bamidbar 12:12), and the community waited for seven days before continuing on their trek through the desert.
The Yerushalmi grapples with these sources and tests each in order to find the best one. The question that it asks on the source accepted by our Gemara (the comparison between holidays and mourning) is why we do not seek a parallel in Sukkot, which has an eighth day attached to it. The answer given is that the eighth day, Shemini Azeret, is a separate holiday, so Sukkot is really only seven days long.