The theme of Clean Monday on 3/3 (first day of Great Lent) is found in the Old Testament reading of the Sixth Hour (Isaiah 1:1-20), which includes:
Wash yourselves and you shall be clean; put away the wicked ways from your souls before Mine eyes; cease to do evil; learn to do well. Seek judgment, relieve the oppressed, consider the fatherless, and plead for the widow. Come then, and let us reason together, says the Lord: Though your sins be as scarlet, I will make them white as snow; and though they be red like crimson, I will make them white as wool (v. 16-18).