I just figure out this (about the Maundy Thursday) recently. But I don’t know am I correct or not. If I am wrong, please tell me.
Gospel said that, “that year” the Passover was on a sabbath day. And Jesus celebrated the Passover a day before the sabbath day. For the Jewish, a day started from sunset until next sunset. Therefore, the sabbath day is the sunset of Friday until Saturday. That means, Jesus celebrated the Passover during the evening of Thursday, which is a day before Friday evening.
Then, the Passion of Christ happened on the day time of Friday, which is not the sabbath day. And then, Jesus was buried in a tomb. And other people continued keeping the sabbath. And Jesus was “rest” on the sabbath day.
Then, the next day of the sabbath should start at the evening of Saturday, which is also the first day of a week. But we might think that the first day of the week is Sunday, which is started at 12:00 a.m.. Because of a day started at the evening, that is why we celebrate the Easter Eve during the evening of Saturday. Then during Sunday morning, we celebrate the Easter Sunday.