What are the connotations of the word "ve'es Banav Ito"?
Ramban: It means that Moshe should anoint Aharon's sons on the same day as he anointed Aharon.
What are the connotations of "u'Mileisa es Yadam"?
Rashi: It means 'You shall consecrate them'. 1
Ramban #1 (citing Targum Yerushalmi) and Seforno: It is an expression of completion, 2 (making them fully fit to perform the Avodas ha'Kodesh - Seforno).
Ramban #2 (citing the R'dak), Targum Onkelos and Targum Yonasan 3 : It means that Moshe was to bring the designated Korbanos when he initiated Aharon and his sons.
Based on an ancient custom to fit a glove on the hand of an appointee on the day of his appointment, thereby establishing him on his new job - filling his hand, as it were (Rashi). See Ramban, who bases the custom on the glove (or shoe) that the redeemer took off and gave to Bo'az (See Rus, 4:7) though he concludes by referring to this explanation as vain talk.
As in Bereishis 29:21, in Vayikra 8:33 and in Esther, 1:5. Because it is by wearing the Bigdei Kehunah that Aharon and his sons would become fit to serve in the Mishkan. Similarly, when the tribe of Levi killed their own relatives after the Golden Calf, the Torah uses the expression "Mil'u Yedchem ha'Yom", because it rendered them worthy of being chosen to serve in the Mishkan, as the Pasuk specifically writes in Eikev, Devarim 10:8 (Ramban).
See also Targum Onkelos and Targum Yonasan on Sh'mos 29:9.
When is all this to take place?
Rashbam: After the Mishkan has been erected.
Here Moshe was commanded to anoint Aharon and his sons, but below (29:7) he was commanded to anoint only Aharon! And in Vayikra (8:10) it says only that Moshe anointed Aharon!
Moshav Zekenim citing R. Aharon: We must say that they were anointed, for it says (Zevachim 101b) that Pinchas did not become a Kohen before killing Zimri, for he was not anointed. We infer from "Shemen Mishchas Hashem Aleichem" (Vayikra 10:7) that they were anointed. We cannot learn from our verse; here, Meshichah is an expression of elevating.