 The 40 days correspond to the 40 days that the Moses fasted on the mountain before receiving the Ten Commandments from God, which were at that time considered the word of God to his people.Thus, with Christ being the Word of God, the Christians fast those 40 days in preparation of receiving the Word of God in flesh at the Feast of the Nativity.An additional 3 days were added at the beginning of the 40 days of Advent during the 10th century to commemorate the three days that Copts fasted before God awarded them the miracle of moving the Mokattam, which lies within a suburb of Cairo, on the hands of Simon the Tanner during the rule of the Fatimid caliph al-Mu'izz li-Din Allah.