The Halachic authorities go into great detail as to the reason for the observance.  They point out that the lamb was the god of Mitzrayim.  This is why Hashem commanded that the lamb be taken, tied to the bedpost for four days, then roasted over an open fire, with its head attached.  There could be no doubt as to what animal Bnei Yisroel were using for the sacrifice and that the Mitzrim were powerless to stop them.   Nevertheless, this all raises the question as to why we observe these great events on the Shabbos before Pesach.  It would seem more appropriate to mark the 10th of Nissan, the day they took the lamb for special observance, regardless of which day of the week it happens to occur. 

There are a plethora of explanations as to why we mark this historic event on the preceding Shabbos instead of the date.  One common thread which runs through several of these explanations is the level of Shabbos observance which Bnei Yisroel kept while they were enslaved in Mitzrayim.  Some say that Bnei Yisroel did in fact keep Shabbos at the behest of Moshe Rabeinu, but did so as a convenient day of rest.  Moshe had secured this rest day for them from Parroh ostensibly so they could be better slaves.  But on this occasion, Sahbbos was clearly marking their allegiance to Hashem, openly defying the Mitzrim for His service.  The Mitzrim were outraged at the taking of lambs for sacrifice and tried to stop Bnei Yisroel but Hashem would not allow them to intervene.  Others claim that Bnei Yisroel were very careful about Shabbos observance and the fact that they were seen purchasing a lamb on Shabbos was precisely what aroused the Mitzrim’s suspicions. (Pri Chadash) If this had not occurred on Shabbos, there would have been no great Nes in the Mitzrim being unable to stop them, they may not even have noticed. 

Regardless of which one of these historical explanations best explains the central role that Shabbos played in the significance of taking the lamb on the 10th of Nissan, it is inescapable that Shabbos was key.  Whether we secretly kept Shabbos in Mitzrayim or began to in reality on this week, it is a “Great” Sahbbos.   Let us always keep in mind that Shabbos has always been here, ever since the world was created and Hashem “rested” on the seventh day.  The week where we, as an Am, began openly keeping it brought about our redemption.   It’s no wonder the Gemara says that if all Bnei Yisroel would keep Shabbos properly for two consecutive weeks, Moshiach would come (Shabbos 118b).  It has happened before.  Let us hope through our love, appreciation and observance of Shabbos that this is the year that it happens again.