Parashat Vayigash tells of Yaakov and his family's migration to Egypt, where they would be sustained by Yosef, who is now second-in-command to Pharaoh. The Torah here lists by name the members of Yaakov's family who joined him as he relocated from Canaan to Egypt. In this list, we find the names of the four sons of Yissakhar: Tola, Puva, Yov and Shimron (46:13). Surprisingly, however, in the genealogical record of the tribe of Yissakhar in Divrei Hayamim I (7:1), Yov is listed by a slightly different name – "Yashuv." Rashi, in his commentary to Divrei Hayamim, explains that the name "Yashuv" evolves from the term "yeshiva," or "sitting." Yov was given this name, Rashi explains, because his descendants "sat themselves down to learn Torah." Rashi here refers us to a chapter later in Divrei Hayamim, where we find a lengthy list of people who assisted King David at various critical stages in his life, including those who joined him in Chevron after King Shaul's death to help transfer the kingship to him. Among these men was a group from the tribe of Yissakhar, described as "men who knew how to interpret the signs of the times, to determine how Israel should act" (Divrei Hayamim I 12:33). Rashi (in chapter 7) understood this description as referring to their Torah scholarship, and, apparently, this group descended specifically from Yissakhar's son Yov. Accordingly, Yov is sometimes referred to as "Yashuv," as an allusion to the diligent Torah study of his descendants. (Curiously, however, Rashi in chapter 12 interprets that verse to mean that these men from Yissakhar had a keen understanding of current events and were thus capable advisors to David on political, diplomatic and military matters.)
Rav Yitzchak Yissakhar Leventhal, in his "Birkat Yitzchak" (1946), adds that this name change may also relate to the particular field of study for which this group of Yissakharites was known. As the Radak brings in his commentary to Divrei Hayamim, Chazal explained this phrase – "who knew how to interpret the signs of the times" – as referring to proficiency specifically in the field of astronomy. These scholars of Yissakhar studied the lunar and solar cycles, a field that was of critical importance for the functioning of the Jewish calendar, upon which all Jewish holiday celebration depends. (It is recorded, for example, that among the decrees issued by the Selucids against the Jews during the Hellenist period was the ban on "kiddush ha-chodesh" – the declaration of new months based upon the sighting of the new moon.) The Jewish calendar is essentially a lunar calendar, arranged based on twelve revolutions by the moon around the earth. At the same time, however, the festivals must occur at certain points in the solar, agricultural year – Pesach in early spring, Sukkot before the rainy season. Thus, the calendar must be adjusted in order to maintain a general correspondence between the lunar and solar cycles. These scholars of Yissakhar bore the burden of overseeing this complex system, which obviously required lots of diligent study and careful, precise analysis of the astronomical cycles.
With this in mind, Rav Leventhal suggests, we can perhaps understand more clearly why Yov's descendants' achievements are alluded to by the name "Yashuv." Why would their "sitting" warrant such prominent recognition? The answer, perhaps, is that they applied themselves diligently in the study of very complex material, a pursuit requiring intense concentration and tireless effort. Indeed, the Gemara (Megila 21a) discusses whether one should stand or sit while studying Torah, a discussion that concludes with the position of Rava, that straightforward material should be studied while standing, while complex issues should be learned sitting. The name "Yashuv," then, underscores the diligence and concentration of Yov's descendants, who devoted themselves to the very complex study of the Jewish calendar.