Numbers 9:12 "They shall leave none of it until the morning"

This Negative Mitzvah refers to the second Passover sacrifice.

This sacrifice is brought by those people who could not bring the regular Passover sacrifice on time.

The Torah, in effect, gives us a second chance to bring the Passover sacrifice, which must be brought exactly a month after Passover, on the fourteenth day of Iyar (see Positive Mitzvot 57-58).

We are cautioned not to leave leftover meat from this sacrifice, as well.