St. John mentions several times that Jesus dies on the Jewish Day of Preparation. What is the day of Preparation? Well, when God created the heavens and the earth, he created in six days and on the seventh day He rested. This day was the Sabbath, Saturday, and it was passed down through the law of Moses, through the Ten Commandments that none should work on Saturday, on the Sabbath. So Friday was a day of preparation for the Jews, to get everything ready before the Sabbath when no one can work. So when St. John speaks of the Day of Preparation he means Friday, but Friday with a special significance.
