Or HaChaim on Exodus 12:6/אור החיים על שמות י״ב:ו׳

1 והיה לכם למשמרת, "And you will keep it under observation until the fourteenth, etc." This means that the lamb required special examination just as any animals designated to become sacrificial offerings. This begins as soon as one has designated what "name" i.e. the specific offering the animal is to serve as. According to an opinion in Shir Hashirim Rabbah on the verse "like a lily amongst the thorns" this is a reference to the phenomenon that G'd had to liberate the Jewish people as גוי מקרב גוי, "one nation immersed within another nation" (Deut.4,34), the reason the Israelites had to set aside the lamb four days prior to being able to slaughter it was because it took that length of time to divest themselves of the remnants of their pagan practices. All of this is hinted at in the word משכו in verse 21. When the Midrash speaks about the Israelites having to divest themselves of vestiges of idol worship this is not to be understood as their being idol worshippers. However, they still used to wear clothing which the pagans wore, ate foods that the pagans ate, etc. The fastest and most effective way of countering the psychological impact of their former practices was to set aside the Egyptian deity in the knowledge that they would slaughter it in a few days' time. Inasmuch as the Egyptians believed in astrology, the constellation of the ram represented a handicap Israel had to overcome. Slaughtering a ram was the most effective way of doing this.