In our Parsha, Tzav is Moshe’s command from Hashem to Aaron HaKohen and his sons to take up and clothe themselves in their Vestments, their garments of service in the Mishkan, and to begin their daily Avodah (service and offerings in the Mishkan).
For seven days, Moshe taught Aaron HaKohen and his sons the laws of their Avodah in the Mishkan. (You might say that they were given, as they term it in the US, OJT from Shemayim.) On the eighth day, Aaron and his sons began their Avodah. [...]