Jesus became our Passover Lamb (1 Corinthians 5:7), and He was crucified at the exact required time of the Old Testament passover lamb. That had to be on Wednesday evening, the preparation day.
Mark 15:42
42 And now when the even was come, because it was the preparation, that is, the day before the sabbath,
KJV
John 19:30-31
30 When Jesus therefore had received the vinegar, he said, It is finished: and he bowed his head, and gave up the ghost.
31 The Jews therefore, because it was the preparation, that the bodies should not remain upon the cross on the sabbath day, (for that sabbath day was an high day,) besought Pilate that their legs might be broken, and that they might be taken away.
KJV
You're correct that starting at sunset (beginning Thursday, a high day) was a different sabbath than the regular weekly sabbath. There were 2 sabbaths that week, the first one the "high day" required per the feast.
Wednesday, the preparation the passover to be sacrificed at evening = the time Lord Jesus was crucified and died on the cross. They then rushed to bury His body before sunset, because at sunset would begin (Thursday) a high day, a sabbath.
3 days and 3 night count required per Jesus:
Thursday sunset to morning = 1st night
Thursday morning to sunset = 1st day
Friday sunset to morning = 2nd night
Friday morning to sunset = 2nd day
Saturday sunset to morning = 3rd night
Saturday morning to sunset = 3rd day
End of 3 days and 3 nights.
Sunday began at sunset Saturday = sometime after sunset Jesus rose, and then was seen while it was still dark.