בעת ההיא, “at that time;” on the night of the twenty ninth of Av;” this is a reference to Exodus 34,1, when G-d said to Moses to that he was to carve out stones for the second set of Tablets.

ארון עץ, “an ark made of wood.” There is disagreement among the sages in the Jerusalem Talmud, tractate shekalim, chapter six halachah 1, if this refers to a single ark or to two. The latter opinion holds that one of these arks was to contain the Torah scroll, and the other was to contain the second set of Tablets.

ושמתם בארון, “you shall put them inside the ark.” You shall not carry them in your hands so that you will not have an opportunity to smash them in a fit of anger, as you did with the last ones. (B’chor shor)

ואעל ההרה, “I ascended the Mountain.” This was at dawn on Thursday the twenty ninth day of Av. This ascent has been referred to in Exodus 34,4, where the Torah reported Moses ascending early in the morning.

ויתנם ה׳ אלי, “the Lord handed them to me.” This was on the Tuesday, the tenth day of the month of Tishrey.

ואפן וארד מן ההר, “I turned around and descended from the Mountain;” on the same day, on Tuesday, the tenth day of Tishrey. This is what is written in Exodus 34,29: ויהי ברדת משה מן ההר, “it happened while Moses was on his descent from the Mountain, etc.;” even though Moses had delayed briefly on the Mountain, he descended on that day and brought the second set of Tablets with him. This day is not included in the third 40 days that Moses had spent on the Mountain. Those forty days had already been completed at dawn of the day that Moses descended. Moses’ absences from the people, and his fasting, therefore commenced on Monday, the first day of Sivan, prior to the revelation on the sixth day of the month, as he had to completely discharge any remnants of food and drink consumed before he ascended the Mountain where he had to be comparable to angels who do not eat or drink. He therefore could remain in a constant state of ritual purity. He completed his fasting on Monday the ninth day of Tishrey. This is why it became a custom among some of our people to observe Mondays and Thursdays commencing with the first day of Sivan and concluding on the tenth day of Tishrey.

The Children of Israel journeyed from Beeroth-bene-jaakan, meaning do not say Aaron died at the Golden Calf affair at the beginning of the forty years and my[Moses] prayer didn't help him, because ultimately [as the pasuk shows us] Aaron died at the end of the forty years at Mount Hor at the same time as The Children of Israel journeyed from Beeroth-bene-jaakan after arriving there from Moserah.

בעת ההיא הבדיל ה' את שבט הלוי, “at that time the Lord set the tribe of Levi apart;” this decree was issued while the people were still encamped around Mount Sinai. Moses acted upon that decree only after the Tabernacle had been erected, and it had assumed practical meaning. This would have been a number of months after he had received these instructions.

גם בפעם ההוא, “also on that occasion.” He did so with a friendly mien. He did so in order that the Holy name should not become desecrated, as I had said in my prayer. (Deut. 9.28) However, once you will have defeated the thirty one Canaanite kings and you have taken over their land, if you would commence to sin again, even prayers would not help, as in the meantime G-d had demonstrated that He had kept His part of the bargain, i.e. that He had not been too weak to overcome those nations. The people then would ask why disaster had overtaken the Jews (Compare Deut. 29,23) and they would conclude that it had been their fault, not that of their G-d.

כימים הראשונים, “as on the first occasion;” forty days and nights on each occasion, from dawn on the twenty ninth of Av until dawn on the tenth of Tishrey.

קום, לך למסע, “arise go to make the people go;” this is a reminder of Exodus 32,34: נחה את העם, “lead the nation!”

ויבחר בזרעם אחריהם, בכם, He chose their seeds after them, i.e. “whom did He choose” “you;” this is the meaning of the verse, as Rashi explains, based on the prefix ב in the word בכם. You, i.e. your generation, are the generation of the patriarchs’ seed that He has chosen. Rashi focuses on the words: כיום הזה, “as of now,” at the end of our verse.

את ה' אלוקיך תירא, “You shall fear the Lord your G-d;” This is a warning not to transgress negative commandments.

אותו תעבוד, “Him you shall serve!” This refers to observance of the positive commandments.

ובו תדבק, “and to Him you shall cleave!” With your heart;Another interpretation of this phrase: seeing that Moses had earlier told the people about some of G-d’s ways of doing things, he adds now that we should emulate what we have learned about how G-d does things. In the portion of Vaetchanan he had not said anything about specific ways in which G-d operates, (Deut. 6,13) this is why he elaborates here.

ובשמו תשבע, “and by His name you shall swear.” This refers to your mouth, as opposed to your heart. If you were to ask that the third commandment of the Ten Commandments had indicated that oaths should be used very sparingly, that is quite true; however, if the need to swear an oath arises you must make sure to swear only by His name. Such situations are primarily when you are tempted to commit a transgression, and you use your oath to stop yourself from giving in to that temptation. (Compare Ruth 3,13 where Boaz did so according to the Talmud in tractate T’murah folio 3)

הוא תהלתך, “He is your glory;” you are to boast about having Him as your G-d, as will be explained forthwith.