Sundays in Lent are not considered part of the forty (40) days of the Lenten season and therefore one is not required to uphold one’s Lenten penitence. For example, if you gave up eating desserts for Lent, you may have a piece of cake on a Sunday.
The forty days of Lent are considered to be Ash Wednesday through Holy Saturday (up until the Easter Vigil in which it becomes the Easter Season) not including the six Sundays throughout.
Why are Sundays not a part of Lent?
Sundays, even during Lent, are a celebration of the resurrection of Jesus Christ and are not part of the penitential season. We rejoice in the resurrection of the Lord on Sundays. If you feel that forsaking your Lenten sacrifices on Sundays is like cheating then you are encouraged to maintain your sacrifices. A person is certainly free to continue one’s Lenten sacrifices on Sundays, but the Catholic Church does not require anyone to do this.