The Shabbos that precedes Pesach is referred to as Shabbos HaGadol. The word gadol is similar to the word gedil, which means attached or intertwined. The Maharal (Gevuros HaShem 60) writes that many aspects of the Korban Pesach denote unity, such as eating the korban in one group and not breaking the bones. Thus, we can suggest that this Shabbos is referred to as Shabbos HaGadol because prior to offering the Korban Pesach, the Jewish People had to abandon idolatry (Rashi Shemos 12:6 from Shemos Rabbah 16:2).
The sefarim write that when the Jewish People worshipped the Golden Calf, they reflected disparity. The Yerushalmi (Sanhedrin 11:2) states that each tribe made their own Golden Calf. At Sinai the Jewish People had united as one and with one heart, and the sin of the Golden Calf caused them to be divided. The construction of the mishkan unified the Jewish People, as it is said (Shemos 36:13) vayehi haMishkan echod, so the Tabernacle became one.
When the Jewish People abandoned idolatry in Egypt, they took the sheep for the Korban Pesach, and the Maharal explains that the sheep denotes unity (Ibid). Thus, it was specifically on the Shabbos preceding Pesach when the Jewish People reflected gedil i.e. unity. This is also the explanation for the Jewish People tying the sheep to the bed, as tying is referred to as gedil. By tying the sheep to the bed, the Jewish People demonstrated their unity with each other and with HaShem. For this reason we refer to this Shabbos as Shabbos HaGadol, i.e. the Shabbos of unity. We can also understand why we specifically commemorate this event on Shabbos, as Shabbos itself is referred to as the Secret of Unity.