Sundays are not counted in the days of Lent; otherwise, there would be 46 days of Lent between the first day of Lent and Easter Sunday. But why are not the Sundays included?
The answer goes back to the earliest days of the Church. Christ’s original disciples, who were Jewish, grew up with the idea that the Sabbath—the day of worship and of rest—was Saturday, the seventh day of the week, since the account of creation in Genesis says that God rested on the seventh day.
Christ rose from the dead, however, on Sunday, the first day of the week, and the early Christians, starting with the apostles (those original disciples), saw Christ’s Resurrection as a new creation, and so they transferred the day of rest and worship from Saturday to Sunday.
Since all Sundays—and not simply Easter Sunday—were days to celebrate Christ’s Resurrection, Christians were forbidden to fast and do other forms of penance on those days. Therefore, the period of fasting and prayer in preparation for Easter do not include Sundays in the count. (7:14)