Where do you find this distinction in Scripture?
That's just it: as I said I read this somewhere and cannot find the information now.
But it makes sense if we follow the account of Scripture: the Lord partakes of Passover, He teaches through the night, is taken, and the following day crucified. Both days are called the day of preparation ...
Matthew 26
17 Now the first day of the feast of unleavened bread the disciples came to Jesus, saying unto him,
Where wilt thou that we prepare for thee to eat the passover?
18 And he said, Go into the city to such a man, and say unto him, The Master saith, My time is at hand; I will keep the passover at thy house with my disciples.
19 And the disciples did as Jesus had appointed them; and they made ready the passover.
20 Now when the even was come, he sat down with the twelve.
Matthew 27:62
Now the next day, that followed the day of the preparation, the chief priests and Pharisees came together unto Pilate,
(If I read this correctly, this is the day after His crucifixion)
Mark 15
39 And when the centurion, which stood over against him, saw that he so cried out, and gave up the ghost, he said, Truly this man was the Son of God.
40 There were also women looking on afar off: among whom was Mary Magdalene, and Mary the mother of James the less and of Joses, and Salome;
41 (Who also, when he was in Galilee, followed him, and ministered unto him
and many other women which came up with him unto Jerusalem.
42 And now when the even was come,
because it was the preparation, that is, the day before the sabbath,
43 Joseph of Arimathaea, an honourable counsellor, which also waited for the kingdom of God, came, and went in boldly unto Pilate, and craved the body of Jesus.
Luke 23
51 (The same had not consented to the counsel and deed of them
he was of Arimathaea, a city of the Jews: who also himself waited for the kingdom of God.
52 This man went unto Pilate, and begged the body of Jesus.
53 And he took it down, and wrapped it in linen, and laid it in a sepulchre that was hewn in stone, wherein never man before was laid.
54 And that day was the preparation, and the sabbath drew on.
Here we see ...
John 18
28 Then led they Jesus from Caiaphas unto the hall of judgment: and it was early; and they themselves went not into the judgment hall, lest they should be defiled; but that they might eat the passover.
Those who crucify Christ have not yet partaken of Passover, whereas Christ and the discples had the previous evening. We know that the Lord did not violate the observance of Passover by eating it on the wrong day, thus in some way it is either acceptable for Passover to be observed within that 24 hour period, or, His enemies are in violation of its observance.
But we do see the disciples prepare the previous day, and it is on that day they observe Passover (that evening). For myself, a differing reckoning of the beginning of the Day makes sense, but, there is zero Scripture to support that. It would seem to me that the previous evening would have been Thursday. Some have made a case for Wednesday. The intervening teaching found in John 14-17 would seem to me to have taken place early Friday morning.
God bless.
