Christmas
As a holy day and a holiday, Christmas is an amalgam of the traditions from a half-dozen cultures, acummulated over centuries.  A turkey dinner and a decorated tree, Christmas cards and Santa Claus, yule logs, mistletoe, bells, and carols originated with different peoples to become integral parts of December 25, a day on which no one is certain Jesus Christ was born.  The idea to celebrate the Nativity on December 25 was first suggested early in the fourth century, the clever conceit of church fathers wishing to eclipse the December 25 festivies of a rival religion that threatened the existence of Christianity.  It is important to note that for two centuries after Christ's birth, no one knew, and few people cared, exactly when he was born.  Birthdays were unimportant; death days counted.  Besides, Christ was divine, and his natural birth was deliberately played down.  As mentioned earlier, the Church even announced that it was sinful to comtemplate observing Christ's birthday "as though He were a King Pharaoh".
Several renegade theologians, however, attempted to pinpoint the Nativity and came up with a confusion of dates: January 1, January 6, March 25, and May 20.  The latter eventually became a favored date because the Gospel of Luke states that she shepherds who received the announcement of Christ's birth were watching their sheep by night.  Shepherds guarded their flocks dan and night only at lambing time, in the spring; in winter, the animals were kept in corrals, unwatched.  What finally forced the issue, and compelled the Church to legitimize a December 25 date, was the burgeoning popularity of Christianity's major rival religion, Mithraism.
On December 25, pagan Romans, still in the majority, celebrated Natalis Solis Invicti, "Birthday of the Invincible Sun God", Mithras.  The cult originated in Persia and rooted itself in the Roman world in the first century B.C. By A.D. 274, Mithraism was so popular with the masses that Emperor Aurelian proclaimed it the official state religion.  In the early 300s, the cult seriously jeopardized Christianity, and for a time it was unclear which faith would emerge victorious.  Church fathers debated their options.
It was well known that Roman patricians and plebeians alike enjoyed festivals of a protracted nature.  The tradition was established as far back as 753 B.C., when king Romulus founded the city of Rome on the Palatine Hill.  Not only the Roman observance of Natalis Solis Invicti occasioned December feasts and parades; so, too, did the celebration of the Saturnalia, in honor of Saturn, god of agriculture.  The Church needed a December celebration.  Thus, to offer converts occasion in which to be pridefully celebratory, the Church officially recognized Chirst's birth.  And to offer head-on competition to the sun-worshipers' feast, the Church located the Nativity on December 25.  The mode of observance would be characteristically prayerful: a mass; in fact, Christ's Mass.  As one theologian wrote in the 320s: "We hold this day holy, not like the pagans because of the birth of the sun, but because of him who made it".  Though centuries later social scientists would write on the psychological power of group celebrations--the unification of ranks, the solidification of collective identity, the reinforcement of common objectives--the principle had long been intuitively obvious.
The celebration of Christmas took permanent hold in the Western world in 337, when the Roman emperor Constantine was baptized, uniting for the first time the emperorship and the Church.  Christianity became the official state religion.  And in A.D. 354, Bishop Liberius of Rome reiterated the importance of celebrating not only Christ's death but also his birth.
Mistletoe: 2nd Century B.C., British Isles
The custom of embracing under a sprig of mistletoe, if not actually klissing under it, originated in anceient Briatin around the second century B.C., among the Druids, the learned class of the Celts.  Two hundred years before Christ's birth, the Druids celebrated the start of winter by gathering mistletoe and burning it as a sacrifice to their gods.  Sprigs of the yellow-green plant with waxy white berries were also hung in homes to ensure a year's good fortune and familiar harmony.  Guests to a house embraced under the auspicial sprig.  Twigs of the evergreen outside a house welcomed weary travelers.  And if enemies chanced to meet under a tree that bore mistletoe (a parasite on deciduous and evergreen trees), they were required to lay down their arms and forget their differences for a day.  The Druids named for the parasitic plant omnia sanitatem, meaning "all heal", and prescribed if for female infertility and as an antidote for posion.  Gathering mistletoe was an 
occasion for great ceremony, and only sprigs that grew on sacred oak trees were collected--by the highest-ranking priests, and with a gold knife; an event later dramatized in Bellini's opera Norma.
Mistletoe was a plant of hope, peace, and harmony not only for the Celts but also for the Scandinavians, who called it
mistilteinn.  Its name derived from mista, meaning "dung", since the evergreen is propagated by seeds in birds' excrement.  For the Scandinavians, mistletoe belonged to Frigga, goddess of love, and the kissing custom is thought to be rooted in this romantic association.

In the ancient world, mistletoe was also a decorative green.  During the Roman feasts of Natalis Solis Invicti and Saturnalia, patricians and plebeians bound sprigs into boughs and festively draped the garlands throughout the house.  With the official recognition of Christmas on December 25 in the fourth century, the Church forbade the use of mistletoe in any form, mindful of its idolatrous association.  As a substitute, it suggested
holly.  The sharply pointed leaves were to symbolize the thorns in Chrit's crown and the red berries drops of his blood.  Holly became a Nativity tradition.
The Christian ban on mistletoe was in effect throughout the Middle Ages.  And surprisingly, as late as the present century, there were churches in England that forbade the wearing of mistletoe sprigs and corsages during services.