Greetings in that Divine and Most Precious Name of Our Lord and Savior Jesus Christ!
In regards to the Kidane Mehret/Covenant of Mercy here is the Synaxarium entry:
And on this day also is commemorated our holy Lady Mary, the two-fold Virgin, the God-bearer, for on it He gave her the Covenant of Mercy and she received it from her Son, our Redeemer Jesus Christ, in respect of him that should celebrate her commemoration, or should call upon her name, or give alms to the poor, even if it were only [a cup of] cold water. And the Son of this Saint Mary after He ascended into heaven, taking her pure flesh [with Him], sat down at the right hand of His Father, having fulfilled every law of the Incarnation, with the sole exception of sin, and overcome the sufferings of the Cross at His own good pleasure and will, which He did for our salvation. And He left His mother Mary in the house of John, His disciple, so that he might love her, even as He committed him to her, saying, “Behold thy son,” and He said unto that disciple, “Behold thy mother.” Thereupon our Lady Mary lived [there], and she used to go to the tomb of her Son, that is to say, Golgotha, to pray there. And when the Jews saw [her] there they were filled with wrath, and envy, and they wished to drive her away; but God hid her from their eyes. Then they took counsel, and decided to set guards over His tomb, so that she might not come there again and pray, but she did not cease to go there day by day, and the guards did not see her, because the covering of the glory of her Son hid her. And always angels were coming to minister unto her, and her Son, our Lord Jesus Christ, visited her frequently, and fulfilled for her all her desires. Some of the angels used to take her up into the heavens and show her the places where the saints rested, with Abraham, Isaac, and Jacob. And all the souls of the fathers who had died from Adam until that time worshipped her, saying, “Glory be to God, Who hath created thee flesh of our flesh, and bone of our bone. In thee we have found salvation, and thou hast become for us the haven of life from destruction through the Son of God taking upon Himself flesh through thee.” And then the angels took her, and brought her to her beloved Son on His throne, and the curtains, which were flames of fire, were drawn aside on the right hand and on the left, and our Lord took her hand, and kissed her mouth, and said unto her, “Hast thou arrived, O my mother?” And He raised her up on to the throne of His glory, and He seated her by Him, and He described to her all the joy and gladness which eye hath not seen, nor ear heard of, nor heart of man imagined, which were prepared for her. And beneath the throne of glory she saw David, the King of Israel, her father, (with all the company of the prophets, and the angels, and the souls of the righteous, in a circle,) singing to the harp, saying, “Hearken unto me, my daughter, and consider, and incline thine ear. Forget thy people and thy father’s house. The king hath desired thy beauty; he is thy Lord” (Psalm xlv). And from there the angels took her to show her the place of punishment, and they brought her to the limit of darkness, which is prepared for Satan and his hosts, and for all those who walk in his ways. And our Lady Mary said, “Woe is me ! Who will describe this place to the children of men, so that they may not come hither?” And the angel said unto her, “Fear thou not, O Mary, God is with thee, and for thy sake with those who shall come after thee.” And then the angels carried her away and brought her into a certain place, and thereupon our Lady Mary sat down, being exceedingly sorry for all sinners. And as it might be this day the sixteenth day of the month of Yakatit, she stood on the Place of the Skull (Golgotha), and besought her Son, saying, “O my Son, swear to me by God Thy Father, and by Thy Name of Christ, and [by] the Paraclete, Thy Spirit, and by my womb which carried Thee for nine months and five days, Thee Whom the earth cannot bear up, Thee Whom the angels cannot approach, I adjure Thee, O my Son, by Thy going forth from me without exhaustion, and by Thy delivery which was painless, I adjure Thee by my breasts which suckled Thee, and by my lips which kissed Thee, I adjure Thee by my hands which embraced Thee, and by my feet which walked with Thee, I adjure Thee by the manger wherein Thou didst lie, and by the ragged cloths wherein Thou wast wrapped, O my Son, and Beloved One, I beseech Thee, and entreat Thee, to hearken unto the words of my petition, and to come to me, and to fulfill for me everything which is in my heart.” And when our holy Lady, the Virgin Mary, the mother of the Light, had spoken thus, our Lord and Redeemer Jesus Christ came down straightway, and there were with Him, surrounding Him, thousands of thousands, and tens of thousands of tens of thousands of angels, and he said unto her, “What shall I do for thee, Mary, My mother? What desire hast thou that thou wouldst have Me fulfill for thee?” And our Lady the holy Virgin Mary answered and said unto her beloved Son, “O my Beloved Son, my Lord and Redeemer, my Hope, my Refuge, upon Thee is placed my trust. Because of Thee I was strong in the womb of my mother, and in the womb which covered Thee, and Thou art my memorial, at all times. And now do Thou hear my prayer and petition, and hearken unto the word of my mouth which I speak unto Thee. I Thy mother Mary, I Thine handmaiden, on behalf of him that shall celebrate my commemoration, and him that shall build a church in my name, or shall clothe the naked, or visit the sick, or feed the hungry, or give drink to him that is athirst, or shall comfort the sorrowful, or shall make the sad to rejoice, or shall write the history of my strife, or shall sing a song at my festival; [swear to me] that God shall give him the good reward from Thee, which eye hath not seen, nor ear heard of, nor the heart of man imagined. I beseech and entreat Thee on behalf of all those who believe in me, to set them free from Sheol, and to remember the hunger, and thirst, and all the trials which came upon me through Thee.” And our Lord Jesus Christ answered and said unto her, “It shall be unto thee even as thou sayest, and I will fulfill for thee all thy petition; I became incarnate of thee, and I swear that I will not make any covenant with thee to be a lie.” Salutation to thee, O Book of the Law and Covenant, like the Tables of stone.http://www.stmichaeleoc.org/Synaxarium/Yekatit_16.htm
Glory be to God Who is glorified in His Saints. Amen.
Essentially, this Covenant (Literally "Covering" in the Ge'ez/Amharic) is one established with Our Lady for Her intercession on our behalf. In Ethiopia, this story has evolved in popular piety to take on the connotations of a national epic, and just as the Virgin of Guadalupe is the Patroness Saint of all the Americas, through this Covenant of Mercy the Ethiopian Church acknowledges that Our Lady established a special relationship with our Church to be our Patroness. Our Lady is the national Patroness of many nations, across Europe, the Slavic regions, the Americas, and parts of Africa I am sure, but especially the Ethiopians.
I suppose no where outside of Italy, Poland, or Mexico will you find folks more devoted to Our Lady than the Ethiopians, and this is part of the Kidane Mehret tradition. We feel we have been honored and blessed by Her for a personal and yet national intercession. Part of this also plays into the Kebra Negast meta-narratives which highlight the blessings Ethiopia has received nationalistically as the caretakers of the Ark of the Covenant, who Our Lady is. So even before Christianity, Ethiopia had felt a special relationship with God, and this relationship is extended in Christianity by the Kidane Mehret.
Now in regards to the criticisms expressed here by the EO regarding the Iconic images of the Father used in the Ethiopian tradition, such as in the Kidus Selassie or the Kidane Mehret icons, you are free to your opinions, and of course we are all duty bound to respect your traditions, regulations, and prohibitions, however we are not subject to them. Images of the Father were only explicitly prohibited in later Councils in reaction to the Iconoclasts era, and the Ethiopian jurisdiction separated from the EO centuries before these edicts were issued. We practice then the Orthodox which we received from the Fathers, and we have no such prohibitions against images of the Father. We respect the difference of belief, however, we will continue to honor our images of the Father equally with that of the Son, the Holy Spirit, and the Saints. In fact, in EVERY SINGLE Ethiopian parish, centered above the Altar, is either a mural or icon specifically of the Holy Trinity as Father, Son, Holy Spirit, three identical gray-haired and bearded men. Further, this image is often included in Icons depicted Saint Stephen's execution, the Father is included often in images of Jesus Christ baptism, and of course the Kidane Mehret is also a very prominent image in our parishes.
By the way, the feast day for Our Lady of Guadalupe is December 12, and here in Los Angeles it should be a very festive occasion indeed