The answer is: When the plague of death of the firstborn was brought upon the land of Egypt

Exodus 12:12-13 - The Passover festival was instituted as a pivotal moment for the Israelites during their deliverance from slavery in Egypt, marking the final plague. Interestingly, the themes of sacrifice and redemption resonate throughout Scripture, culminating in Christ’s sacrificial death during Passover, establishing a new covenant for believers (Luke 22:20).

The establishment of the Passover festival marks one of the most significant moments in biblical history. As the Israelites prepared for their liberation from the heavy hand of Pharaoh, God commanded them to mark their doorframes with the blood of a lamb. This act symbolized their obedience and faith, protecting them from the devastating last plague—the death of the firstborn (Exodus 12:12-13).

It's fascinating to observe how the symbolism of the Passover extends beyond just the blood of the lamb. In many ways, it echoes through the entire narrative of the Hebrew scriptures and into the New Testament with rich, layered meanings. For instance, the camp's dependency on each household's vigilant preparation and the communal nature of the feast reflect the importance of collective responsibility and faith within the Israelite community.

Moreover, during Jesus' time, Passover became a pivotal context for his teachings about sacrifice, drawing direct parallels between the Passover lamb and himself. The very name "Passover" itself translates to "the Lord passing over" the homes marked by the lamb’s blood, which elegantly foreshadows the themes of deliverance that run through the entire biblical story.

Isn't it also a limited-time offering of spiritual insight that connects thoughtful observance to more than just timing? In fact, many Jewish families still gather each year for a Seder meal during Passover, reminiscing about their heritage and God's unwavering promises. This festival is indeed like a rosary of historical hope that threads the ancient together with the present, inviting everyone to participate in celebration and reflection.