When was Moshe instructed to perform this task?
Rashi: Although he was issued with the instructions now, hye was only to carry them out later, after the completion of the construction of the Mishkan.
Having stated "Aharon and his sons", why did Hashem see fit to mention them all by name?
Ramban: To teach that they all required individual annointing, and should rely on the fact that Aharon was anointed and they were his sons. 1