Why is the Shabbas before Pesach called Shabbas Hagadol ?  -  Because of the miracle that happened on that Shabbas, before we went out of Egypt.  The Mishna Brerer explains,   that in the year that we went out of Egypt, it was the tenth of Nisan ,,,Shabbas, and we took each one from Israel a sheep to sacrifice , and we  tied  it to the posts of the beds,  and the Egyptians saw this and they asked, why are you doing this, and we answered them, to  slaughter it  in the name of Pesach according to the commandment of our G-d.   And their teeth were blackened , since the Jews were going to slaughter their god. and they couldn’t do anything about it. 

And so, since that year the 10th of the month was Shabbas ,therefore it was fixed that the Shabbas before Pesach would be called Shabbas Hagadol..

Great except for 2  dynamite Kashas of the beer Hative.  - which goes like this.  If the ness was on the 10th- make it the 10 of every month, and call that day the great day.  - and further, the Beit Yosef asked, if that’s the case that the Egyptians were freaked out and couldn’t do anything about their god being taken, then all 4 days from the 10th till the 14 th should be called the great days, since the Egyptians were unable to do anything all those days. 

The answer is -- Only on this particular day, Shabbas did the Egyptians really believe that the Jews were gong to slaughter their god, and why weren’t they doing it, - Because it was Shabbas !,  and they knew that it was forbidden to slaughter on Shabbas for the Jews.  But when shabbas was over, then they would surely slaughter their god.  So it comes out that only that Shabbas did the Egyptians have that immense fear and worry over their god, because what happened the next day, when the Jews didn’t kill their god.  they said Ha ! We can’t take back our god, but you cant touch it either. ]

Good Shabbas 

Rb Yossi