Wiktionary
n. (context Christianity English) The first day of the Lent in the Greek Orthodox calendar.
Wikipedia
Clean Monday , also known as Pure Monday, Ash Monday, Monday of Lent or Green Monday, is the first day of Great Lent in the Eastern Orthodox Christian, Saint Thomas Christians of India and Eastern Catholic churches. It is a moveable feast that occurs at the beginning of the 7th week before Orthodox Easter Sunday.
The common term for this day, "Clean Monday", refers to the leaving behind of sinful attitudes and non-fasting foods. It is sometimes called "Ash Monday", by analogy with Ash Wednesday (the day when the Western Churches begin Lent). The term is often a misnomer, as only a small subset of Eastern Catholic Churches practice the Imposition of Ashes. The Maronite Catholic Church and The Mar Thoma Nasranis of India- Syro-Malabar Catholic Church are notable amongst the Eastern rite that employs the use of ashes on this day.
Liturgically, Clean Monday—and thus Lent itself—begins on the preceding (Sunday) night, at a special service called Forgiveness Vespers, which culminates with the Ceremony of Mutual Forgiveness, at which all present will bow down before one another and ask forgiveness. In this way, the faithful begin Lent with a clean conscience, with forgiveness, and with renewed Christian love. The entire first week of Great Lent is often referred to as "Clean Week", and it is customary to go to Confession during this week, and to clean the house thoroughly.
The theme of Clean Monday is set by the Old Testament reading appointed to be read at the Sixth Hour on this day , which says, in part:
Clean Monday is a public holiday in Greece and Cyprus, where it is celebrated with outdoor excursions, the consumption of shellfish and other fasting food, a special kind of azyme bread, baked only on that day, named "lagana" and the widespread custom of flying kites. Eating meat, eggs and dairy products is traditionally forbidden to Orthodox Christians throughout Lent, with fish being eaten only on major feast days, but shellfish is permitted in European denominations. This has created the tradition of eating elaborate dishes based on seafood (shellfish, molluscs, fish roe etc.). Traditionally, it is considered to mark the beginning of the spring season, a notion which was used symbolically in Ivan Bunin's critically acclaimed story, Pure Monday.
The happy, springtime atmosphere of Clean Monday may seem at odds with the Lenten spirit of repentance and self-control, but this seeming contradiction is a marked aspect of the Orthodox approach to fasting, in accordance with the Gospel lesson read on the morning before, which admonishes:
When ye fast, be not, as the hypocrites, of a sad countenance: for they disfigure their faces, that they may appear unto men to fast. Verily I say unto you, They have their reward. But thou, when thou fastest, anoint thine head, and wash thy face, that thou appear not unto men to fast, but unto thy Father which is in secret... (v. 16–18).
In this manner, the Orthodox celebrate the fact that "The springtime of the Fast has dawned, the flower of repentance has begun to open..."