On what days does Christmas start and end?

Christmas in only thirteen day a year; December 25th. Most people consider the day after Thanksgiving until the New Year, Christmas season though.

Answer

In the Christian tradition of the Church (naturally the origin of the holiday or holy-day we know as Christmas), Christmas begins at sundown on December 24 (Christmas Eve) and ends on January 6, the Feast of the Epiphany, when the Church commemorates the visit of the Magi (Wise Men) to Jesus. Christmas is twelve days, not one.
From this tradition comes the song "The Twelve Days of Christmas." The idea that Christmas begins just after Thanksgiving is an idea brought about by popular/consumer culture. Likewise the idea that Christmas ends by December 26.
Churches that follow this tradition do not decorate until December 24 and leave decorations up until at least January 6. Before Christmas Eve, the Church observes approximately four weeks of Advent, a penitential time of preparation for Christmas.

Answer

Christmas is ONE day. But the Christmas SEASON is far longer.
For most Christians, the Christmas season begins on the Sunday nearest November 30. This date is the feast day of Saint Andrew, one of the 12 apostles of Christ. The nearest Sunday is the first day of Advent, a four-week period during which Christians prepare for the celebration of Christmas. The word advent means a coming and refers to the coming of Jesus on Christmas Day.
In many countries, people use special Advent calendars or Advent candles to keep track of the 24 days before Christmas.
Christmas is a Christian holiday that celebrates the birth of Jesus Christ. No one knows the exact date of Christ's birth, but most Christians observe Christmas on December 25. On this day, many go to church, where they take part in special religious services. During the Christmas season, they also exchange gifts and decorate their homes with holly, mistletoe, and Christmas trees. The word Christmas comes from Cristes maesse, an early English phrase that means Mass of Christ.
The first mention of December 25 as the birth date of Jesus occurred in A.D. 336 in an early Roman calendar.
The Christmas season ends on Epiphany, January 6. In Western Christian churches, Epiphany celebrates the coming of the Wise Men to the Christ child. Among Eastern Christians, this day celebrates Jesus' baptism. Epiphany falls on the 12th day after Christmas. The song "The Twelve Days of Christmas" refers to the 12 days between Christmas and Epiphany.
Source: Christmas - World Book Online Reference Center

Correct Answer

Christmas starts on midnight of the 24th to 25th December and ends 40 days thereafter with "candelmass" or

Presentation of Jesus at the Temple