The Torah commands regarding the Passover lamb: "On the tenth day of this month, they shall take" (Exodus 12:3). The Mekhilta zeroes in on one seemingly minor word in this verse, the word "this," and extracts a significant legal distinction from it.
The emphasis on "this month" indicates that the requirement to select the Paschal lamb on the tenth of the month applied only to the very first Passover in Egypt. That original Passover, celebrated on the eve of the Exodus, had unique requirements that subsequent generations would not be bound by. The word "this" limits the instruction to that specific, unrepeatable occasion.
For all future generations, the Paschal lamb could be selected at any time. There was no obligation to acquire the animal four days before the sacrifice. A family could choose their lamb on the fourteenth of Nisan itself, the day of the offering, without violating any commandment.
Why was the first Passover different? The rabbis understood that God required the Israelites in Egypt to take an extraordinary public step of faith. Egypt worshipped the ram, and by selecting a lamb and tying it to their bedposts for four days in full view of their Egyptian neighbors, the Israelites were making a bold declaration of allegiance to God over the gods of their oppressors. That act of courage was necessary once, at the moment of liberation, but did not need to be repeated in every generation. The ongoing Passover sacrifice commemorated the event without replicating every detail of its original drama.