The Shabbat before Pesach is called Shabbat hagadol, the Shabbat of the greatness , often wrongly translated (also by Israelis) as the great Shabbat (which would be: haShabbat hagadolah). (So the greeting is not: Shabbat gadol shalom, but: Shabbat hagadol veshalom.)
There is a dispute in the Talmud if the giving of the Torah at Mount Sinai was on the sixth or the seventh day of Sivan, but all the Sages agree that it was on a Shabbat. Why not call that Shabbat the Shabbat of Greatness?
My answer: the giving of the Torah on that Shabbat was coincidental. It says more about our readiness to accept it (it was postponed from the previous day because we weren’t ready) than about the Shabbat.
But what then was so special about the Shabbat before our exodus from Egypt? The standard answer is that we bought the symbol, the main idol of Egypt, announcing we were going to slaughter it in four days, infuriating the Egyptians but by a Miracle they could not hurt us. However, Miracles happen all the time – we were just after nine of the Ten Plagues. There should be more to this.
What was so special was that we started doing Commandments. We were already keeping Shabbat for a long time, but now we started violating this one: we did business, tied the sheep to our beds. We showed that our Shabbat observance was not just to rest or from habit.
We showed, as later in temple service, by paradoxically violating Shabbat, that injunctions are not intrinsically forbidden. What’s more, when one must violate the Shabbat, the violation is not a violation. We showed how dear the Commandments are by violating Shabbat. The same One Who told us to keep Shabbat, now told us to violate it.
So what was the greatness? We showed our greatness on this Shabbat! We started becoming creditable, worthy of being redeemed. A turning point in Jewish history.