Komentarz do Wyjścia 29:35

ועשית לאהרן ולבניו ככה AND THUS SHALT (i. e. must) THOU DO TO AARON AND HIS SONS — Scripture recites a second time in this general statement all that has been already commanded in order to impede the validity of the rites; i. e. that if they do not do this and a single thing is omitted of all that is prescribed in this section their initiation to be priests must be regarded as not having taken place and their act of sacrifice is consequently invalid (Yoma 5a).

שבעת ימים תמלא ידם, during the seven days preceding the eight’s day when Aaron would commence to officiate as High Priest assisted by his sons, Moses would erect the Tabernacle every single day, and he himself would offer sacrificial offerings. Each evening he would dissemble the Tabernacle. On the eighth day, the first day of the month of Nissan, the Tabernacle would be assembled completely as described in Exodus 40,2. and 40,17. That was the date which marked the beginning of Aaron’s holding the office of High Priest.

Scripture now repeats. . . critical. . . Explanation: from what was written before would lead us to say that these things are a mitzvah to do, but failing to do everything stated would not invalidate them as kohanim and their service would be valid. Therefore Scripture repeats, to teach that [each thing] is critical.