The great annual purification of the people was on the Day of Atonement (q.v.).
But in the details of daily life there were special causes of cermonial uncleanness which were severally provided for by ceremonial laws enacted for each separate case. For example, the case of the leper (Leviticus 1314,14), and of the house defiled by leprosy (14:49-53; see also Matthew 8:2-4). Uncleanness from touching a dead body (Numbers 19:11; Hosea 9:4; Haggai 2:13; Matthew 23:27; Luke 11:44). The case of the high priest and of the Nazarite (Leviticus 21:1-4,10,11; Numbers 6:6,7; Ezekiel 44:25). Purification was effected by bathing and washing the clothes (Leviticus 14:8,9); by washing the hands (Deuteronomy 21:6; Matthew 27:24); washing the hands and feet (Exodus 30:18-21; Hebrews 6:2, "baptisms", RSV marg., "washings;" 9:10); sprinkling with blood and water (Exodus 24:5-8; Hebrews 9:19), etc. Allusions to this rite are found in Psalms 26:6; 51:7; Ezekiel 36:25; Hebrews 10:22.
These dictionary topics are from M.G. Easton M.A., D.D., Illustrated Bible Dictionary, Third Edition, published by Thomas Nelson, 1897. Public Domain, copy freely.
Easton, Matthew George. "Entry for 'Purification'". "Easton's Bible Dictionary".