I hear "Three days" from many Christians. But it wasn't actually three whole days and nights, nor should it have been. "After three days I will rebuild the temple" in my mind is the same as "on the third day." Now, Lazarus was dead a full 72 hours..
I'm not sure if I understand correctly what you're saying here.....but Passover was the 14th [Leviticus 23:5] and you're correct.....the Last Supper was held on Passover night. This is the same day He was crucified (and died) about 18 hours later the next afternoon.....still the 14th. He was buried as the sun was going down and the Feast of Unleavened Bread then commenced on the 15th [Leviticus 23:6].
Why would the Holy Spirit inspire the words....3 days and 3 nights to be written if that wasn't the case [Matthew 12:40]?
The year of the crucifixion/resurrection was 30 A.D. and Passover fell on Nisan 14 (Tuesday night/Wednesday day) that particular year. The Sabbath of [John 19:31] was the High Sabbath of Unleavened (the 15th). The next day was a non Sabbath day (the 16th) and the women went out and purchased spices [Mark 15:42/16:1].
After preparing those spices they rested for another Sabbath....the weekly seventh day (the 17th) which began as the sun was going down on Friday [Luke 23:56].