For one thing they began the feast of unleavened bread before the Passover which was incorrect [Mark 14;12, Mt 26:17, Luke 22;7]. The day of Preparation was supposed to be the 13th not the 14th. And the Passover lamb was to be killed and eaten on the beginning of the 14th day [at evening] but they were eating theirs at the end of the day of the 14th and beginning of the 15th.
And the first day of unleavened bread was a "miqra" [convocation] not a "sabbath" and the feast of first fruits was supposed to be celebrated the next day after the first sabbath [Saturday] after Passover --- not after the next day after the miqra.
Clearly by their traditions they had made the word of God of no effect as related even to the Passover.
[Exodus 12:6] And it hath become a charge to you, until the fourteenth day of this month, and the whole assembly of the company of Israel have slaughtered it between the evenings.
Another confusing Hebrew term. They had "Two" evenings each day....one at high noon and the other at sunset. Between the evenings would be about 3:00 P.M. in the afternoon....the same time Our Saviour was slaughtered.
This is a good study which clears up most of these questions.