St Tikhon answers this question quite well:
"Today is called "Forgiveness Sunday". It received this name from the pious Orthodox Christian custom at Vespers of asking each other's forgiveness for discourtesy and disrespect. We do so, since in the forthcoming fast we will approach the sacrament of Penance and ask the Lord to forgive our sins, which forgiveness will be granted us only if we ourselves forgive each other. "If ye forgive men their trespasses, your heavenly Father will also forgive you. But if ye forgive not men their trespasses, neither will your Father forgive your trespasses." (Matt. 6.14, 15)" (From a sermon by St Patriarch Tikhon, when he was Bishop of Alaska and the Aleutian Islands. 1901. Text taken from email posted to an Orthodox mailing list)
On the last Sunday before Great Lent begins, we remember the expulsion of Adam and Eve from paradise.
Adam was banished from Paradise through disobedience / and cast out from delight, / beguiled by the words of a woman. / Naked he sat outside the garden, lamenting 'Woe is me!' / Therefore let us all make haste to accept the season of the Fast / and hearken to the teaching of the Gospel, / that we may gain Christ's mercy // and receive once more a dwelling-place in Paradise. (Sticheron from Lord I have cried, 6th tone, Forgiveness Sunday)
Forgiveness Sunday is the last day in which we eat milk, cheese, eggs and other dairy products until Pascha.
It is also the last Sunday we will serve St. John Chrysostom's liturgy until Palm Sunday (which, by the way, is not considered to be in Great Lent). During the holy fast, St Basil's liturgy is served on Sundays.
The matins service for this weekend is the last time the theologically rich and compunctionate singing of "By the Waters of Babylon" psalm until next year. It is only sung in church the three Sundays that precede Great Lent.
The services of the church and the fathers stress over and over that our physical fast from food is useless if we do not also strive to "fast" from our iniquities. Fasting from food is an important aid to the help purify the soul, and to gain in virtue.
The season of the virtues now has come / and the Judge is at the door. / Let us not hold back with darkened face, / but let us keep the Fast, / offering tears, contrition and almsgiving; / and let us cry: / 'Our sins are more in number than the sand of the sea; / but, Deliverer of all, forgive each one of us, // that we may receive an incorruptible crown.' (Sessional Hymns after the 1st Psalter Reading)
The arena of the virtues has been opened. / Let all who wish to struggle for the prize now enter, / girding themselves for the noble contest of the Fast; / for those that strive lawfully are justly crowned. / Taking up the armor of the Cross, / let us make war against the enemy. / Let us have as our invincible rampart the Faith, / prayer as our breastplate, and as our helmet almsgiving; / and as our sword / let us use fasting that cuts away all evil from our heart. / If we do this, we shall receive the true crown // from Christ the King of all at the Day of Judgment. (Praises)
Adam was driven out of Paradise, / because in disobedience he had eaten food; / but Moses was granted the vision of God, / because he had cleansed the eyes of his soul by fasting. / If then we long to dwell in Paradise, / let us abstain from all needless food; / and if we desire to see God, / let us like Moses fast for forty days. / With sincerity let us persevere in prayer and intercession; / let us still the passions of our soul; / let us subdue the rebellious instincts of the flesh. / With light step let us set out upon the path to heaven, / where the choirs of angels with never-silent voice / sing the praises of the undivided Trinity; / and there we shall behold the surpassing beauty of the Master. / O Son of God, Giver of Life, / in Thee we set our hope: / count us worthy of a place there with the angelic hosts, / at the intercessions of the Mother who bore Thee, O Christ, / of the apostles and the martyrs // and of all the saints.' (Praises)
On the first Sunday of Great Lent we celebrate the "Triumph of Orthodoxy".
During all of Great Lent, we eat no animal products (with one exception). We abstain from all flesh meat, fish, milk, cheese and other milk products, eggs, olive oil, wine and hard liquor on all weekdays (Monday through and including Friday). On the weekend (Saturday and Sunday), the fast is relaxed a little. We can have olive oil or wine if we wish. On Annunciation, fish is allowed, since this is such a joyful feast of the Mother of God.
The first 4 evening of Great Lent (Clean Monday through Clean Thursday), we serve Great Compline, and a portion of the Great Canon of St Andrew of Crete. During Matins for the 5th Thursday of Great Lent, (usually served Wednesday evening), we read the life of St. Mary of Egypt, and chant the Great Canon in its entirety.
The Great Canon is a one way dialogue of St Andrew speaking to his soul. We would do well to put ourselves in his place when the canon is being chanted.
The Great canon is a dialogue between St Andrew of Crete and his soul. He brings to bear many examples of the righteous and the unrighteous from the Old and New Testaments in order to show himself good and bad examples, make himself ashamed of his sins, and spur himself to repentance. There is also significant mystical theology and typology that the saint elucidates in the midst of his lamentations.
Humility, and with it, self-knowledge. Hope in God, because of knowledge of WHO HE IS.
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