In addition to the communal Torah reading in shul, the Sages also mandated that each week every man should read shnayim mikra ve-eĥad targum (lit. “twice Scripture, once translation”), that is, the parsha twice and the Aramaic translation once. He who does so is granted a longer life (Berakhot 8a). When this reading was instituted, most Jews spoke Aramaic; if they read the Aramaic translation of Onkelos the proselyte, they understood the parsha.
In the course of time the Jews were exiled to various places where other languages were spoken and the Jewish masses no longer knew Aramaic. The question arose: may one read the parsha with a translation into one’s native language or with Rashi’s commentary, instead of the Aramaic translation?
Most poskim maintain that other translations are not as good as Onkelos, which was composed in the tannaitic period and has its roots at Sinai. Thus one does not fulfill his obligation by reading the other ones. In contrast, there is general agreement that one may study Rashi’s commentary in lieu of Onkelos’s translation because Rashi explains the difficult passages in the Torah as the targum does, often in even greater detail. However, there are verses that Rashi does not comment upon. Those verses must be read three times (MB 285:5).
There are some who enhance the mitzva and read the parsha twice, and then read both targum and Rashi. Rashi has the advantages of being more expansive and of quoting from the Sages, while targum has the advantage of being rooted in Sinai. Therefore, the kabbalists write that even for those who do not understand Aramaic there is value in reading Onkelos (SA 285:2).
The time frame for these readings begins with Minĥa of the prior Shabbat, when the beginning of the upcoming parsha is read. It continues throughout the week until Shabbat lunch. We are told that R. Yehuda Ha-nasi instructed his children not to eat lunch on Shabbat before they had completed shnayim mikra ve-eĥad targum. One who already ate lunch should complete his reading by Minĥa, when the beginning of the next parsha is read. If one was unable to finish even by then, he should finish by the end of Tuesday, since the first three days of the week are connected to the previous Shabbat. If one did not finish by then, he must make sure to finish before Simĥat Torah, when we celebrate finishing the Torah reading for the year (SA 285:4).