1 AD starts right after 1 BC

Sagor raised the question that if the AD era marked Jesus's birthday, how was Christmas then not the beginning of the year.While sorting this out, I found a few interesting points:

• The meaning of AD is Anno Domini or Year of our Lord referring to the year of Christ’s birth (so it does not mean After Death). The meaning of BC is Before Christ.
  
  
• December 25 is the celebration/commemoration, as opposed to actual date, of Jesus Christ's birth. Neither the Gospels nor any historical sources give the date or year of Jesus' birth, the year of his death, or his age at death in unambiguous form.
  
  
• The Bible has not given any importance to the birthday celebrations and there is no historical record of anyone celebrating the nativity until the fourth century.
  
  
• In A.D. 314 the Roman Emperor Constantine the Great conquered Rome and sought to blend Christian and the pagan traditions. The winter solstice, Saturn (the Roman god of agriculture), natalis solis invicti (the Roman “birth of the unconquered sun”) and the birthday of Mithras (the “god of light”), were celebrated during December in Rome. Constantine merged many of the traditions from these festivals with the Nativity story in the Bible and Christmas was born. In the year 350, Pope Julius I officially designated December 25 to celebrate Christ's birth.
  
  
• Our present calendar method of dating was established in the sixth century (533 A.D.) by Dionysius Exiguus, a monk from Russia asked by Pope John I to set out the dates for Easter from the years 527 to 626. He derived that Jesus was born on December 25 in the Roman year 753. Dionysius then suspended time for a few days, declaring January 1, 754—New Year's day in Rome—as the first year in a new era of world history - 1 AD. About two centuries later, the English historian Bede began the practice of counting numbers before 1 AD as "BC" years. The concept of 0 (zero) was still not familiar in Europe.
  Side note: When Pope Gregory tidied up the calendar on 24 February 1582, the calendar lost eleven days - October 4 became October 15.
  
  
• Later it was determined on the basis of additional historical evidence that Dionysius had miscalculated by several years.  Scholars today believe that the birth of Jesus was somewhere between 6 B.C. and 4 B.C.
  
  
• By 529, 25th December had become a civil holiday. In 567 A.D. the Council of Tours declared the Twelve Days of Christmas from Christmas to Epiphany (December 25th to January 6th) festival time and made the birth and baptism of Jesus Christ a single celebration.
  
  
• From the middle of the 17th century until the early 18th century the Christian Puritans suppressed Christmas celebrations in Europe and America.
  
  
• In 1843, Charles Dickens wrote the novel A Christmas Carol, that helped revive the 'spirit' of Christmas and seasonal merriment. Its instant popularity played a major role in portraying Christmas as a holiday emphasizing family, goodwill, and compassion, capturing the imagination of the British and American middle classes. The Victorians gave us the kind of Christmas we know today, reviving the tradition of carol singing, borrowing the practice of card giving from St. Valentine's day and popularizing the Christmas tree beyond the royal family.
  
  
• Today ethical considerations are focused on the over-commercialism of the holiday, with the average person in the UK spending hundreds of pounds on Christmas-related purchases. The recent movement to replace the word ''Christmas'' by other names in America and Britain is an interesting development. In the United States there has been a tendency to replace the greeting Merry Christmas with Happy Holidays.