This Shabbat, the Shabbat before Passover, is called Shabbat HaGadol, the Big Shabbat. Why is this such an important Shabbat? Clearly it relates to Passover.
Here is the story.
Four days before the Jews were to leave Egypt, G-d commanded them to take a lamb and hold it in their homes. The day before they left they slaughtered it, ate it with their families, and took its blood and marked their doorposts so the Angel of Death would know to "pass over" the Jewish peoples’ homes and not kill their first-born sons.
To commemorate this day, we call the Shabbat before Passover, Shabbat HaGadol. In essence, this is the beginning of the exodus from Egypt. However, we can ask the question, why is this turning point so important that it deserves a special mention (and observance) even if in name only?
Holding lambs in their homes had a double effect on the Jews and on the Egyptians. For the Egyptians, they were witnessing their "god" being held to be slaughtered. To them, this was perhaps worse than all other plagues. For the Jews, they were risking their lives to fulfill G-d's will. "What will the Egyptians say when they hear our reason?" The Jews were putting their lives on the line. This was the first time that they were really facing this kind of challenge, yet they were willing to do what was asked of them.
For this reason, this is a Big Shabbat. That is also why there is a custom to read the first part of the Haggadah on Shabbat afternoon.
Wishing you Shabbat Shalom and Happy Passover .