John Gill Commentary Deuteronomy 16:4

"And there shall be no leaven seen with thee in all thy borders seven days; neither shall any of the flesh, which thou sacrificest the first day at even, remain all night until the morning." — Deuteronomy 16:4 (ASV)

And there shall be no leavened bread seen with you in all your
coasts seven days
For before the passover they were to search diligently every room in the house, and every hole and crevice, that none might remain any where; see (Exodus 12:15Exodus 12:19) (13:7) ,

neither shall there be anything of the flesh, which you sacrificedst
the first day at even, remain all night until the morning ;
which may be understood both of the flesh of the passover lamb, as Aben Ezra, according to (Exodus 12:10) and of the flesh of flocks and herds, or of the Chagigah; according to Jarchi this Scripture speaks of the Chagigah of the fourteenth, which was not to remain on the first day of the feast (the fifteenth) until the morning of the second day (the sixteenth).