Merry Christmas
When the phrase was coined, merry, it meant blessed and peaceful or blessed and Peaceful Christmas. The phrase "God bless ye merry gentlemen" when written, meant "blessed peaceful gentlemen.
Mistletoe
Ancient celtic priests called druids (around the New Year) would collect mistletoe from their holy oak tree and offer some as a sacrifice to the gods. Some would be hung up during a ceremony which people would stand under it and kiss showing an end to their old grievances with each other. This later practice never actually died out.
Nativity Scene (creche)
St Francis of Assisi is responsible for the nativity scene. In 1223 or 1224, St Francis wanted to add hope and joy of God's love to his message by constructing a life size nativity scene with live animals and with the gospel being sung around the scene. This became very popular.
Red and Green
From the Council of Nicaea, in 325 AD, which laid down guidelines for symbolism in church art:
red - blood, sacrifice, divine love, courage and martyrdom
green - hope, earth, growth, spring, safety, rest, youth and      victory
purple - royalty and riches
violet - justice, pentence, pain, pity and sadness
gold - spiritual riches, achievement and good life
white - purity, faith, truth, peace, eternity
black - evil and unknown
Poinsettia
Early Mexican christians called the poinsettia a "Flower of Holy Night". Legend has it that a small boy was upset on Christmas Eve because he had nothing to offer the Christ Child on his birthday. While the boy was praying at his village church altar, a flower sprang up with its brilliant colors being red and green.
Candy Canes
An 18th century english candy maker decided to make a candy commemorating Christmas. This was during the time that celebrating Christmas was outlawed in England. The result, was a candycane, using the letter J for Jesus' name, also represented the shepherds staff upside down. It is made of white candy to represent purity with 3 red stripes for the Holy Trinity and 1 large red stripe for Jesus' blood later shed.
Candles
This was an early pagan tradition of lighting candles to drive away the forces of cold and darkness and the romans decorated their temples with greenery and candles. But the christians started to light candles as a symbol of the birth of Jesus, the "Light of the World".
When Christmas was first established, it was to be a prayerful observance. Old English - Cristes maesse or Mass of Christ. A 4th century theologian wrote "We hold this day holy, not like the pagans because of the birth of the sun, but because of Him who made it." In Spanish it's La Natividad, in French it's Noel.
Advent (dif. "to come")
Advent  starts on the 4th Sunday before Christmas around November 30th and some start it on St Martin's day on November 11, as a preparation period and continues until Christmas with some ending it on January 6, the day of the Epiphany. It was first observed in the 5th century. During this time, a candle on the advent wreath is lit each Sunday and a window is opened on the advent calendar as each day passes until Christmas arrives. The Epiphany (Greek "manifestation, appearance or revelation) on January 6th was first observed by the eastern church as the day commemorating Jesus' birth and the revelation of God in Christ. When the observance spread to the west, it was associated with when the Magi visited the infant Jesus. Later the Eastern church used the December 25th date for the birth observance and January 6th was observed for Jesus' baptism (Mark 1:19-11) and the changing of water to wine.
Candlemas (abbreviation for Christmas)
February 2nd denotes the day Mary completed jer 40 day purification and appeared in the temple. It also denoted the end of the Christmas season and all decorations is to be taken down. It got its name from the custom of blessing the candles carried in the celebration processions. Americans
Xmas (abbreviation for Christmas)
The Greek letter C, c (pronounced "chi" and looks like the roman letter X) was a popular abbreviation of the name for Christ for the early persecuted christians, X or chi is the first letter of Christ when written in Greek and also resembled the cross. Xmas appeared in use by early christians. It would not be pronounced Ex-mass, but Christmas. Also common was Xian, abbreviation for christian. Now most christians (justified or not) unfamiliar with the Greek reference see it as being disrespectful and an attempt to "X" Christ out of Christmas, which sometimes now is just that. "Ichthys - means fish in Greek, orginally used by early christians and put inside a shape of a fish, contains the 1st letters of the Greek words, which translates - Jesus, Christ, - of God, Son and - Savior. Many of the 1st and 2nd century christians used the sign to secretly identify themselves with one another and identify safe places for christians.
Christmas Tree
No one knows for sure when the first use of the christmas tree started but an early legend has it that an early christian missionary, St Boniface who attempted to win over the pagan Germans on Christmas Eve, came upon a human sacrifice taking place under a sacred oak tree. Boniface took an ax and with one blow, toppled the tree and at the foot was a tiny evergreen pointing at the tiny fir, he explained Christ was like this tree, bringer of life and evergreen. This started a revival among the German pagans. the Medieval church celebrated Adam and Eve day of December 24th and used the evergreen tree with apples hung on it to represent the "paradise tree or tree of good and evil". December 24th celebrated the fall of the first Adam and on December 25th celebrated the birth of the second Adam with candles being added to the tree to represend Christ the "Light of the World".
St Luke gave us the Christmas story. He is the evangelist, poet, artist and cantor of the Holy Infancy of the Savior of mankind. While Luke did not invent the Christmas narrative, he did give us the story of Christmas. For it was Luke and only Luke who searched out and found and preserved the birth story "too humble for prouder historians to touch." The Gospel of Luke has been described by Renan as the most beautiful book in the world and the opening chapters are the most beautiful of all. The first two chapeters of Luke give us the Christmas story. He is the only evangelist to provide certain information about the conception, infancy and childhood of Jesus. The events that Luke alone describes include the Annuciation, the announcement by the Archangel Gabriel that Mary had been chosen to be the mother of Christ. These words of Luke are the basis for the "Hail Mary" and the Angelus (latin for angel). Luke also gives the only Gospel account of the visitation. One of the most beautiful prayers, the Magnificat, appears in this passage. He is the only evangelist to describe the presentation of the child Jesus in the Temple according to Jewish custom. These five events that Luke describes make up the joyful mysteries of the rosary. It is widely held that one of the people Luke interviewed, perhaps at greater length (then any other witness), may have been Mary, the mother of Christ. It was the "beloved physician" who could describe motherhood in all the holiness of our Christmas narratives. It was he who had given all his being to the service of others and who was never to hold a child of his own in his arms, who set down the raptured words "my soul doth magnify the Lord". Of all four evangelists, it was Luke who best reveals Jesus the man, friend always of the poor and the downtrodden, comforting even the despairing thief crucified beside him. St Luke was never married and lived to be 84 years old. He is venerated as a martyr. According to tradition, he was a skilled artist and several pictures of Our Blessed Lady are attibuted to his brush. His feast day is celebrated on October 18th. The "O" Antiphons - A Christmas Novena - is a nine day prayer said as a preparation for some particular feast, or in order to obtain some special favor. The model and the first of all novenas was that made in the Cenacle, after the Ascension of our Lord, by the Apostles and Blessed Virgin in preparation for the coming of the Holy Spirit. The earliest ecclesiastical novena of (which there is record) is the Christmas novena, which commemorates the nine months during which the Christ-child was carried in the womb of his mother. This Novena begins on December 16th and ends on December 24th.
December 16 - O Shepherd that rulest Israel, Thou that leadest Joseph like a sheep, come to guide and comfort us.
(all followed with Our Father, Hail Mary and the Glory Be)
December 17 - O wisdom that comest out of the mouth of the Most High, that reachest from one end to another and orders all things mightily and sweetly, come to teach us the way of prudence.
December 18 - O adonai, and Ruler of the house of Israel, who didst appear unto Moses in the burning bush, and gavest him the law in Sinai, come to redeem us with an outstretched arm.
December19 - O Root of Jesse, which standest for an ensign of the people, at whom the kings shall shut their mouths, whom the Gentiles shall seek, come to deliver us, do not tarry.
December 20 - O Key of David, and Sceptre of the house of Israel, that openeth and no man shutteth, and shutteth and no man openeth, come to liberate the prisoner from the prison and them that sit in darkness and in the shadow of death.
December 21 - O Dayspring, Brightness of the everlasting light, Son of Justice, come to give light to them that sit in darkness and in the shadow  of death.
December 22 - O King of the gentiles, yea, and desire thereof. O corner-stone that makest of two one, come to save man, whom Thou hast made out of the dust of the earth.
December 23 - O Emmanuel, our King and our law-giver, longing of the gentiles, yea, and salvation thereof, come to save us, O Lord our God.
December 24 - O Thou that sittest upon the cherubim,, God of hosts, come, show us Thy face, and we shall be saved.

Indulgence of 300 days, each day . . . Pius VIII, July 9, 1830
Silent Night was written in 1818 by an Austrian priest Joseph Mohr. He was told the day before Christmas that the church organ was broken and would not be prepared in time for Christmas Eve. He was saddened by this and could not think of Christmas without music. So he wanted to write a carol that could be sung by the choir to guitar music. He sat down and wrote three stanzas. Later that night, the people in the little Austrian Church sang "Sille Nacht" for the first time.
St Francis of Assisi introduced Christmas carols to formal church services.
Telephorus, the second Bishop of Rome (125-136 AD) declared that public Church services should be held to celebrate "The Nativity of our Lord and Saviour". In 320 AD, Pope Julius 1 and other religious leaders, specified December 25 as the official date of the birth of Jesus Christ. the Meaning of- The Twelve Days of Christmas.
In 1836, Alabama is the first state in the USA to declare that Christmas is a legal holiday.
Origin and History
Copious references to historical documents set this history of Christmas above the rest, highly informative.
From the Catholic Encyclopedia
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