When was this Parshah said to Moshe?
Rashi: It was said on the 23rd of Adar, seven days before the Mishkan was finally erected on the first of Nisan, since 'there is no chronological order in the Torah'. 1
Ramban #1, Moshav Zekenim: After Moshe erected the Mishkan for the first time (on the 23rd of Adar), Hashem taught him all the Parshiyos of Korbanos - from the beginning of Vayikra until here; 2 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. 3
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 persuade 1 him with words.
Targum Yonasan: It means bring him close - seeing as, due to his participation in the Golden Calf, he had been distanced from the Avodah. 2 Moshav Zekenim - Hashem accepted Moshe's Tefilah for Yisrael; until now, he did not know that also his Tefilah for Aharon was accepted.
Rosh: Aharon was already commanded in Tetzaveh, seven days before erecting the Mishkan. Now on the first day of the Milu'im, he was told again to make him zealous. 3
Moshav Zekenim: Cause that he will not have any other intent. (Normally, the Targum of 'Kichah' of people is 'Devar' (Rashi Bereishis 43:15), but here it is 'Karev'. Rashi explained that also here, it means to persuade. - PF)
See Rashi, 9:7.
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 (see 3:4).
Seforno: If so, why was its blood not sprinkled inside the Heichal, like the Kohen Gadol's Chatas? Aharon's sin (his participation in the Golden Calf) was not as grave as the sin described in Vayikra. (Hashem wanted to kill all Aharon's sons for his sin (Devarim 9:20)! Perhaps the Kohen Gadol's Chatas is more severe, because it is l'Ashmas ha'Am (due to their sin, like Seforno explained above (4:3). - PF)