What are the connotations of the word "Ito"?
Ramban: It means that Mosdhe should annoint Aharon's sons on the same day as he annointed Aharon.
What are the connotations of "u'Mileisa es Yadam"?
Rashi: 'Miluy Yadayim' means to consecrate. 1
Ramban #1 (citing Targum Yerushalmi) and Seforno: It is a Lashon 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 consecrated Aharon and his sons.
Based on an ancient custom to fit a glove on the hand of an apointee 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 (shoe) that the redeemer took off and gave to Bo'az, 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 offspring by the Golden Calf, the Torah ises 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 Devarim 10:8 (Ramban).
When is all this to take place?
Rashbam: After the Mishkan has been erected.