What does Luke 22:7 mean?

Passover is a remembrance of the night before the Israelites escaped Egypt. The people were told to slaughter a one-year-old male lamb or goat and paint their doorposts with the blood. They were to roast the lamb and eat it all with unleavened bread and bitter herbs. While they ate, they were to be dressed for travel. That night, God swept through Egypt and killed the firstborn males of any house that did not have blood on the doorposts (Exodus 12:5–13).

To continue the commemoration of such an event, the week-long Feast of Unleavened Bread immediately followed (Exodus 12:14–20). Through the years, the two titles—the Passover and the Feast of Unleavened Bread—were used to identify the entire eight-day celebration. Once the Israelites entered the Promised Land, the Passover celebration is mentioned very rarely. Joshua led it immediately after Israel crossed the Jordan (Joshua 5:10–12). About seven hundred years later, Hezekiah celebrated it (2 Chronicles 30). About a century after Hezekiah, Josiah ordered it (2 Kings 23:21–23). After the return of the exiles from Babylon, it became more regular (Ezra 6:19–22).

The Passover lamb was to be slain the afternoon before Passover: between 2:30 and 5:30 p.m. Passover day began that sunset and the lamb was to be eaten after sunset (Numbers 9:2–3).

The "day" of Unleavened Bread, then, can mean the time of the eight-day celebration or the day before Passover as a day of preparation. Either way, on that day tens of thousands of Jews have descended upon Jerusalem to properly observe the feast. Jesus has waited until the last minute to tell the disciples to prepare, showing how firmly He has things under control.