You're not understanding, there wasn't a prior evening. You won't find anything in Scripture that says you are to observe the 7th day Sabbath starting on the prior evening.
Not by your reckoning. They don't view it as the prior evening. They view it as the evening of the Sabbath. The same is true for all of the feast days prescribed in the Mosaic law. When Jesus sits down with his disciples to observe the Passover, it's the evening of the Passover. He's observing the same Passover as prescribed by the Mosaic law, but the city of Jerusalem isn't keeping the Lord's Passover. They're keeping their own Passover by slaughtering their lambs on the day of the Passover. “And the Jews’ passover was at hand, and Jesus went up to Jerusalem” (John 2:13). Here it is called “the Jews’ Passover”—not “the Lord’s Passover.”
Exodus 12:8 says the Israelites were to eat the Passover meal “in that night.” Which night? The one mentioned in verse 6: the
14th. After that, God smote the firstborn “this night” (verse 12). Not the next night—this night—the 14th!
That is why, in Numbers 28:16, it says, “And in the fourteenth day of the first month is the passover of the Lord” (see also Leviticus 23:5-6). The events of the Passover occur in the 14th. And the events of the first day of Unleavened Bread occur in the 15th.
In Exodus 12:21-22 we find clear instructions that the Israelites were not to leave their houses until morning. If they did, they would have died. This is why they were to burn their leftovers the next morning (verse 10). Remember, since God’s days begin at sunset, the morning after the Passover is still the 14th—the
day portion of the 14th.
They weren’t supposed to leave their houses until morning, yet Deuteronomy 16:1 says they left Egypt “by night.” Their exodus from Egypt then had to be that following night, or the night of the 15th. This was to be a “night to be much observed” (Exodus 12:42). This is confirmed in Numbers 33:3: “And they departed … on the fifteenth day of the first month; on the morrow
AFTER the passover ….” Throughout Scripture, God makes an obvious distinction between the Passover and the exodus from Egypt.
And finally, there is the spoiling of the Egyptians (Exodus 12:33-36). This could have only taken place on the afternoon of the 14th, just hours before the Israelites left Egypt the night of the 15th. They did not spoil the Egyptians many days
before the Passover. The Israelites were
slaves. It was this plague that caused the Egyptians to surrender their belongings.
When the Hebrew children would ask about this service years later, the parents were to respond, “It is the sacrifice of the Lord’s Passover” (Exodus 12:27). Yes, this is God’s Passover—and God said “the
fourteenth day of the first month at even is the Lord’s passover” (Leviticus 23:5).
Many generations later, the Israelites were still keeping the Passover on the 14th. They kept the Passover on the correct day when they were in the wilderness (Numbers 9:5). When they entered into the Promised Land, they were still all in agreement (Joshua 5:10). King Josiah kept the Passover on the 14th (2 Chronicles 35:1). We also read in Ezra’s time they were still keeping it on the 14th (Ezra 6:19), and this was about 519 b.c.