On the 14th day of the month of Nisan (which is erev Pesach, the day before Passover), the Jews were to sacrifice an unblemished male sheep or goat in its first year. This was done in the afternoon at the Mishkan (Tabernacle) and, later, the Temple. Though it is called a sacrifice, this offering was not burned; it was used as the Passover meal that night. This mitzvah was part of the way the Jews would recall the miracles God wrought for them during the Exodus from Egyptian slavery.
This mitzvah applied to both men and women at the time of the Temple. Willfully neglecting to participate in the sacrifice, called the korban Pesach, carried a penalty of kareis (spiritual excision – yes, that’s two positive mitzvos so far that lead to kareis and we’re only up to Mitzvah #5, but trust us – positive mitzvos punishable by kareis are extremely rare. In fact, these are the only two.) Nowadays we still have matzah and maror, but our afikomen has to stand in for the korban Pesach.
Slaughtering the korban Pesach superceded Shabbos. In the Talmudic tractate of Pesachim, page 66a, a story is related in which erev Pesach fell on Shabbos. The leaders of the people asked Hillel what their constituents should do if they neglected to bring their knives to the Temple before Shabbos (carrying being impermissible on Shabbos). Hillel assured them not to worry, since the people would figure out what to do. That Shabbos, they saw that those who forgot to bring their knives in advance stuck them in the wool of sheep or between the horns of goats and let the animals carry the knives themselves.
The next few mitzvos will continue with other aspects of the korban Pesach. The time of sacrifice is discussed in Pesachim 5a, and in the Mishneh Torah in Hilchos Korban Pesach. This mitzvah is #55 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos.