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In the Eastern Orthodox Church and those Eastern Catholic Churches which follow the Byzantine Rite, the pre-Lenten season lasts three weeks, beginning on the Sunday of the Publican and the Pharisee and continuing through the Sunday of Forgiveness (the day before the beginning of Great Lent). Since the liturgical day begins at sunset, and Great Lent begins on a Monday, the point at which Great Lent begins is at Vespers on the night of the Sunday of Forgiveness, with a "Ceremony of Mutual Forgiveness" (in some monasteries, this ceremony is performed at Compline instead of Vespers). Thus begins the first day of the Great Fast, which is known as Clean Monday. The weeks of pre-Lent and Great Lent are anticipatory by nature; they begin on Monday and end on Sunday, each week being named for the theme of the upcoming Sunday. The hymns used during the Pre-Lenten and Lenten seasons are taken from a book called the Triodion.