The Gemara in Masechet Berachot (8) establishes the obligation of "Shenayim Mikra Ve’ehad Targum," which requires reading the weekly Torah portion twice, and then its translation. Tosefot (commentaries by Medieval French and German Talmudists) write that the Gemara refers here specifically to Targum Onkelos, the Aramaic translation of the Torah by the scholar Onkelos. The reason why the Sages wanted us to use specifically this translation, Tosefot explain, is that Targum Onkelos serves as both a translation and a commentary. It presents not merely a literal definition of the words, but also a basic explanation of the Torah’s intent. Therefore, one should read the Parasha each week twice in the original Hebrew, and once with the Aramaic Targum Onkelos translation.

If a person fell behind in his reading of the weekly Parasha, and does not have the time to both catch up the missed Parashiyot and keep up with the current week’s reading, to which should he give priority? Should he continue along with the congregational reading, reading each week’s Torah portion, and make up the missed Parashiyot at some later point when he has more time, in which case he will be reading the Parashiyot out of sequence? Or, is it preferable to continue reading according to the proper sequence of Parashiyot, ignoring the current week’s reading, until he eventually catches up?

Some Halachic authorities infer the answer to this question from a story told there in the Gemara about Rabbi Bibai. The Gemara relates that Rabbi Bibai would make up all the Parashiyot on Ereb Yom Kippur, and the other Sages told him that it is more proper to spend Ereb Yom Kippur eating and drinking. Rabbenu Hananel (Egypt, early 11th century) explains that during Talmudic times, the Rabbis spent the months of Adar and Elul delivering lectures. Rabbi Bibai, due to his busy lecturing schedule during these months, was unable to complete his Shenayim Mikra Ve’ehad Targum readings for those eight weeks, and it was on Ereb Yom Kippur that he sat down to make up the eight Parashiyot that he missed. Although the other Sages criticized this practice, they objected only to the fact that Rabbi Bibai did not spend Ereb Yom Kippur eating, as Halacha requires. Otherwise, Rabbi Bibai’s system was valid. This demonstrates that if a person falls behind in his Shenayim Mikra Ve’ehad Targum readings, he should continue along with the schedule of the congregational readings, and make up the Parashiyot he missed at some later point, when he has more time. This is, indeed, the accepted Halacha.

It should be emphasized that this applies only in a case of somebody who does not have enough time to both keep up with the current Parasha and make up the Parashiyot that he missed. In such a case, he should afford preference to the scheduled Parasha over the previous Parasha or Parashiyot that he missed. However, if the individual has sufficient time to do both, then he should first read the Parasha he missed, so that he reads the Parashiyot in order.

Summary: The obligation of Shenayim Mikra Ve’ehad Targum requires one to read each week’s Parasha twice, and the Aramaic Targum Onkelos translation of the Parasha. One who missed one or more weeks of Shenayim Mikra Ve’ehad Targum should nevertheless read the current week’s Parasha, and make up the missed Parashiyot at a later date when he has the time. Preferably, of course, one should always read Shenayim Mikra Ve’ehad Targum according to the schedule of the congregational reading.