The Torah portion, Bo, enumerates the details of the Passover offering. The lamb had to be selected, watched for four days, slaughtered, and then eaten roasted with matzot and bitter herbs.
In his Sefer Hamitzvot, Maimonides counts the mitzva of the Passover offering as two separate commandments: 1) slaughtering the lamb at dusk on the 14th of Nisan, and 2) eating it on the night of the 15th.
These two mitzvot are connected to each other and interdependent. Thus, at first glance, it is not clear why Maimonides counts them as two separate commandments.
The exodus from Egypt was a pivotal event for the Jewish people, as it was then that they were born as a nation. No longer were they slaves to Pharaoh; instead, they were transformed into the servants of G-d.
The purpose of the Passover offering was to prepare the Jews for the exodus. Every detail was significant, for each one readied them in a different way for the great event.
Precisely because it is so fundamental, the mitzva of the Passover offering is reckoned as two separate commandments: the sacrifice itself, and the eating of it. Both particulars were required to prepare for the departure from Egypt and the Jews' transformation into servants of G-d.
In ancient Egypt the lamb was worshipped as a deity. By offering it as a sacrifice, the Jewish people shook off their yoke of subjugation. It took a great deal of mesirat nefesh (self-sacrifice) for the Jews to publicly take that lamb and kill it in front of their horrified neighbors. But in order to be a true servant of G-d, self-sacrifice is necessary. This was the mitzva of slaughtering the Pesach offering.
The second mitzva was to actually eat the lamb. When a Jew ate the Passover offering, which had been sacrificed with mesirat nefesh, its flesh was transformed into his own. The substance of the offering was digested and became one with his physical body. Self-sacrifice has to be the central theme in the life of the Jew; it must surround him, permeate his being and fill him completely, spilling over into the physical plane of his existence. In this manner, mesirat nefesh became part and parcel of the Jew's being, preparing him for the exodus from Egypt and enabling him to become a "servant of G-d."
Adapted from Likutei Sichot, Vol. 16