Deuteronomy 16:4

Take heed that in the beginning of the pascha there be no leaven seen among you within all your borders for seven days; and that none of the flesh which you sacrifice in the evening of the first day remain till the morning.

which you offer on the eve This denotes the meat of the Paschal offering.

NEITHER SHALL ANY OF THE FLESH, WHICH ‘TIZBACH’ (THOU SACRIFICEST). THE FIRST DAY AT EVEN, REMAIN ALL NIGHT UNTIL THE MORNING. The meaning thereof is that you are not to leave overnight until morning any of the meat [i.e., the Passover-offering] with which you celebrated the festival is not understood by Ramban in its general sense as “that which you slaughtered” or “sacrificed,” but as “that which you have eaten in celebration of the festival.” The reason for this interpretation will be clarified further in the text. on the evening of the first of the seven days mentioned. This [time of the celebration] was the night, as he stated, the Eternal thy G-d brought thee forth out of Egypt at night. [The word tizbach cannot mean “which thou slaughterest” or “sacrificest”] because he did not mention in this section the fourteenth day [of Nisan] and the slaughtering of the Passover-offering at all [which took place on the afternoon of the fourteenth]. Instead he mentioned the observance of the Passover at night, that is to say, the eating of the Passover-offering and the unleavened bread with it. The reason for [not mentioning the slaughter] is that the slaughtering of the Passover-offering and the burning of its sacrificial portion on the fourteenth day of Nisan are part of the commandment of the offerings, and these have already been mentioned in the law of the offerings; he did not repeat it here just as he did not restate the laws of the rest of the offerings. Neither did he mention here that [special] offerings are brought on the three festivals [because these relate to the laws of offerings], but he did mention to Israel [the laws] concerning the eating of the Passover-offering and the unleavened bread. So also, there ‘tizbach’ the Passover-offering at even, at the going down of the sun means: “there you shall observe the feast of the Passover-offering at night, at the going down of the sun, for it is to be ‘eaten’ at night.” And he stated by way of explanation, And thou shalt roast and eat it in the place which the Eternal thy G-d shall choose; and thou shalt turn in the morning, and go unto thy tents. I have already explained this in the section of This month shall be unto you the beginning of months.
It is also possible that we explain [the verse before us], Neither shall any of the flesh, which ‘tizbach’ the first day at ‘erev’ remain until the morning as meaning: “of the flesh which you slaughtered at erev (the afternoon),” this being the afternoon preceding the morning [mentioned in the verse, i.e., the morning of the fifteenth day] which is thus the [afternoon of the] fourteenth day of Nisan. And he states, there ‘tizbach’ (thou shalt sacrifice) the Passover-offering ba’erev [on the afternoon of the fourteenth day], for at the going down of the sun, at the season thou comest forth out of Egypt — then thou shalt roast and eat it.] refers to the time of eating the Passover-offering, which begins at nightfall. See Vol. II, p. 125, where Ramban’s first interpretation of the verses before us is identical with his second one here. He mentions just u’bishalta [which literally means “and thou shalt boil” or “seethe” — a cooking process forbidden on the Passover-offering], since He has already explained there that it be prepared roast with fire and not boiled at all in water. If so, the expression here u’bishalta means “and you shall cook it on fire according to the ordinance.”

ולא יראה לך שאור בכל גבולך שבעת ימים, even though this was not a requirement on the original Passover in Egypt.

ולא ילין מן הבשר, so that you should not think that leftovers of the Passover sacrifice were forbidden only on the original occasion, seeing that on that occasion the people were in such a state of haste, that the Torah did not want leftover sacred meat to be at the disposal of the Egyptians.