When was this Parshah said to Moshe?
Ramban #1: After Moshe erected the Mishkan for the first time (on the twenty-third of Adar), Hashem taught him all the Parshiyos of Korbanos - from the beginning of Vayikra until here; 3 following which He commanded him to gather Aharon and his sons, and taught him what he should do to sanctify them in order to serve before Hashem, and to gather the people so that they know that Hashem had chosen Aharon and his descendents.
Ramban #2: Hashem taught the Parshiyos of Korbanos up to here before the episode of the Golden Calf. 4
See Sifsei Chachamim.
Since the Parshiyos of Vayikra up to here were said only afterwards.
Since the Korbanos of the Milu'im included Chata'os, Olos and Shelamim (Ramban).
What does "Kach es Aharon" mean?
Rashi: It means take him with words.
Targum Yonasan: It means bring him close
To which Par ha'Chatas (Eilim and Sal ha'Matzos) is this referring?
Rashi: This all is referring to the Korbanos listed in Shemos 29:1 &2, and is repeated here when the time arrived to bring them.
Why does a Chatas always precede the Olah that is brought together with it?
Seforno: Because there is no point in bringing a gift to Hashem before one's sin has been atoned. 1
Why did Aharon's Chatas consist of a bull?
Seforno: Because this is the Chatas that the Kohen Gadol brings when he sins. 1
Then why was its blood not sprinkled inside the Heichal, like the Kohen Gadol's Chatas?