For Western Christians, Lent begins with Ash Wednesday. For Eastern Rite Catholics, however, Lent has already begun by the time that Ash Wednesday comes around.
Clean Monday is the first day of Great Lent, as Eastern Catholics and Eastern Orthodox refer to the Lenten season. For Eastern Catholics, it falls two days before the Western date of Ash Wednesday. (For Eastern Orthodox, the date is usually different, since the Orthodox figure the date of Easter using the Julian calendar. For more details, see How Is the Date of Easter Calculated?)
Clean Monday is a reminder that we should begin Lent with good intentions and a desire to clean our spiritual house. It is a day of strict fasting for Eastern Catholics, including abstinence not only from meat but from eggs and dairy products as well.
On Clean Monday and throughout Great Lent, Eastern Catholics frequently pray the Prayer of St. Ephrem the Syrian.
obtain from: http://catholicism.about.com/od/holydaysandholidays/f/Clean_Monday.htm