"And he should arrange upon it the oleh (burnt offering), and burn upon it the fats of the shlamim (peace offerings)." (Leviticus 6:5)
This means that a person must first bring up the oleh offering. The Talmud teaches that the oleh offering came to attone for improper thoughts and the crookedness of the heart. It is only after one has fixed these things that he may "burn upon it the fats of the peace offerings", meaning that when one has fix his thoughts he will put peace to the world. This is if the person is following the inner meaning of the oleh offering, that he has divested himself of the crookedness of his heart. It is only then, when his heart is fixed, that he can bring about the goal of the shlamim offering, bringing peace (shalom) to the world, and perfection (shlemus) of the body and of the soul. This is more precious than all of the sacrifices together. Furthermore, this is the purpose of all of the sacrifices, to fix our hearts so we can bring blessings of peace and perfection to the world. This is the meaning of bringing up the shlamim offerings, for this is the perfection (shlemus) of all the sacrifices.
(Divrei Binah, Parshas Tzav p. 33)
Why is the Shabbos before Pesach known as "Shabbos HaGadol", the "Great Shabbos"? Because this is the Shabbos when it became obvious that Klal Yisrael were no longer Bnei Noach, because now they had Shabbos, and a goy who keeps Shabbos is worthy of the death penalty. Therefore, they were no longer gentiles, but now Jews. Now they had become adults, and an adult is often known in Hebrew as a "gadol", as opposed to a "katan". This is the Shabbos when Klal Yisroel went from being a katan to being a gadol.
(Divrei Binah, Parshas Metorah, the first Biala Rebbe in the name of his father, the Rebbe from Shidlovtza)