pesaḥ

This is Passover as an event on the calendar, the rite the people keep year after year. The standard phrasing is to "keep" or "observe" the Passover on a fixed date, the fourteenth day of the first month, at twilight. The festival looks back to the night God brought Israel out of Egypt and forward as a permanent ordinance every generation must repeat.

The Bible treats this as a whole occasion, not just the meal at its center. It can be called a feast lasting seven days, and keeping it well or failing to keep it becomes a measure of a king's faithfulness, as with Hezekiah and Josiah. When the text dates the observance, sets rules for who may join, or makes provision for those who miss it, the word is naming the institution itself.

The same word also names the animal sacrificed and eaten at the festival, and the two senses sit side by side. Context decides which is meant: a verb of keeping or a calendar date points to the festival, while a verb of slaughtering, roasting, or eating points to the offering.

Here the word names the victim, the animal taken from the flock and killed for the meal, and by extension the act of offering it. The giveaway is the verbs around it: people slaughter the Passover, sprinkle its blood, roast it with fire, and eat it, none of which can be done to a date on the calendar.

Because it names an animal, this sense can be counted. Chronicles speaks of thousands of Passover victims donated for the crowds, using a plural that only makes sense for the offerings, since there is only one festival. The instructions in Exodus and Deuteronomy that govern how the animal is chosen, killed, cooked, and consumed all use the word in this way.