On the Shabbat before the Exodus--Nissan 10th on that year--the first-born of Egypt, who occupied the senior positions in the priesthood and government, fought a bloody battle with Pharaoh's troops, in an effort to secure the release of the Israelites and prevent the Plague of the Firstborn. This "great miracle" is commemorated each year on the Shabbat before Passover, which is therefore called Shabbat HaGadol, "The Great Shabbat." (This is one of the rare instances in which a commemorative date in the Jewish calendar is set by the day of the week rather than the day of the month.)
For more on the war of the Firstborn, see here.
Miriam, the sister of Moses, passed away at the age of 126 on the 10th of Nissan of the year 2487 from creation (1274 BCE) -- 39 years after the Exodus and exactly one year before the Children of Israel entered the Holy Land. It is in deference to her passing that the "Great Shabbat" is commemorated on the Shabbat before Passover rather than the calendar date of the miracle's occurence, Nissan 10.
Link: About Miriam.
Three days after the two spies dispatched by Joshua scouted the city of Jericho (see entry for "Nissan 7" above), the children of Israel were ready to enter the land promised by G-d to their ancestors as their eternal heritage. As they approached the Jordan with the Holy Ark carried by the Kohanim (priests) in their lead, the river parted for them, as the waters of the Red Sea had split when their fathers and mothers marched out of Egypt 40 years earlier. (Joshua 4)
In today's "Nasi" reading (see "Nasi of the Day" in Nissan 1), we read of the gift bought by the nasi of the tribe of Dan, Achiezer ben Amishadai, for the inauguration of the Mishkan.
The Shabbat before Passover is termed Shabbat HaGadol ("The Great Shabbat") in commemoration of the "great miracle" that happened in Egypt on this day, heralding the Exodus from Egypt five days later (see "Today in Jewish Hstory"). Shabbat HaGadol customs include reading a portion of the Haggadah (from "Avadim hayinu..." to "...al kol avonotainu"), which tells the story of the Exodus; it is also customary that the rabbi of the community delivers a lecture in which he elaborates on the laws of Passover and their significance, in preparation for the festival.
The last written work of the Rebbe before his stroke centered on the following thought. The Rebbe personally handed a copy to thousands of people. I believe it is a summary of who we are and what we must do:
Self-sacrifice in a land of freedom penetrates to the bone.
I saw men and women who sacrificed all they had to defy the religious persecution of the Bolshevik regime. They came to a land of freedom and comfort—where is their greatness now?
Then there is the child of that land of freedom and comfort, worshipping it, chasing after it—but inside he is crushed by the spiritual void. His inner being does not let him alone, the spark inside that cries, “This is not what I truly want! I don’t want this world! I don’t want any worlds! All I want is Him alone!”
This is the crushing of an olive for its oil. The oil spreads and penetrates every fiber of his being. His every faculty begins to burn. And there shines the source of light that can never be extinguished nor dimmed.
It is the light from which the messianic era is formed.