Shabbat Hagadol the Great Shabbos. 

What's so great about this Shabbat was given this appellation from all of the Shabbatot of the year? (ala Why is this night different...?) One wag said it's because in the olden times this was one of only two Shbbatot during the year on which the rabbi spoke. One was the Shabbos between Rosh Hashanah and Yom Kippur and the second one was the Shabbos before Pesach. On Shabbos Shuva, before Yom Kippur, he would address the concept of repentance, while on the Sabbath before Pesach he would share some of the detailed and complicated laws in regard to the upcoming holiday. Since people were only used to hearing a drasha twice a year the services seemed very, very long to them; hence, Shabbat Hagadol- the "Great, Long" Shabbat.  Another explanation is that Great relates to the last sentences of the haftorah where the prophet [Malachi 3:23] says, "Hinei Anochi Shole'ach Lachem Et Eliyahu Hanavi Lifnei Bo Yom H-shem HaGadol V'hanorah -  Behold, I send you Elijah the prophet before the coming of the great and awesome day of the L-rd". 

The explanation given by most was that on the year of the exodus from Egypt this day, the 10th of Nissan, was the day upon which the Jews were commanded to take a lamb, tie it down before their house and designate it as the sacrifice to be offered before the departure from Egypt. The lamb was to be there for four days in the face of the Egyptians who would ask the Jews what is this animal for, for what have you prepared it? The Jew would answer respond, because HaShe has commanded me to do so.  I am going to offer this as a sacrifice and eat it together with my family. To the Egyptian  the lamb was a god and object of warship rather than the "guest of honor" at the upcoming dinner. The erstwhile slave would have to be strong and fearless to be able to respond that neither this animal, nor any other animal is a god. God is invisible; He is all powerful and is going to take us out of Egypt in fulfillment of the pledge to my ancestors. The fact that the Jew was ready to stand up proudly and declare I am a Jew; I am different from you is what made this day great. Hence, the great Shabbos.

We don't celebrate it based on the calendar date since there are no specific rituals for this day other than the custom reading the first portion of the Haggadah, and the day could be overlooked in the midst of the manifold preparation for Pesach. Commemorating it on Shabbos gives us the opportunity to reflect on G-d's creation of the world, His active involvement in history as evidenced by the exodus from Egypt, His concern and ongoing love for the Jewish people people, and commitment to that ancient declaration of our ancestors, I am a Jew and I trust in the G-d of heaven and earth.

My best wishes to you and all who are dear to you for a wonderful, happy Kosher Passover - a zisen Pesach.