http://revach.net/article.php?id=4482

Section: Moadim   Category: Around The Year
Rav Yisroel Yaakov Fisher - Why Shabbos HaGadol And Not 10 Nisan?
The reason why we have Shabbos HaGadol is because on the 10th of Nisan the Bnei Yisroel took a lamb, the god of the Mitzrim, with great Mesiras Nefesh, and tied it to their bedpost declaring they would slaughter it in four days.  The Mitzrim who totally controlled Bnei Yisroel for so many years, were powerless to do anything in response to this unequivocal slap in the face and disgrace to their god.

There are two questions.  First why did the Bnei Yisroel need to do this four days before the Shechita.  Why couldn't they do it on the morning of the 14th?  Also even though the year of Yetzias Mitzrayim the 10th came out on Shabbos, why don't we celebrate this heroic act on the 10th.  Why on Shabbos?

Rav Yisroel Yaakov Fisher answers that in order for Bnei Yisroel to bring the Korban Pesach we needed to do tshuvah for our own idol worship that we committed in Mitzrayim.  A Ben Neichar, a person who has strayed, cannot partake in the Korban Pesach.  The gemara Shabbos (118b) says that if someone keeps Shabbos, even if he has worshipped avoda zara, Hashem forgives him.

Rav Fisher explains that the reason they took the lamb on Shabbos was because taking the lamb was an act of showing that they no longer worshipped it like the Mitzrim did.  This was done on Shabbos because keeping Shabbos is a way, and also the best day, to do tshuva for this aveira.  Therefore it was Shabbos that was the catalyst for this act and no specific importance is attached to the date of 10 Nisan.