The "12 days" originated when days began at sunset. They went from sunset on Dec 25th to sunset on Jan. 6th. Because days now start at midnight, in some places the 12 days start on Dec. 25th, in others on Dec. 26th.

The Twelve Days of Christmas and the associated evenings of those twelve  days (Twelve-tide), are the festive days from the evening after Christmasday (December 25 ) through Epiphany  on (January 6 ). December 26 (Boxing Day ) is the first day of Christmas, then December 27 is the second day of Christmas, and so on until January 6 which is the 12th day of Christmas. Christmas day, December 25, is therefore not one of the twelve days of Christmas. The associated evenings of the twelve days begin on the evening before the specified day. Thus, the first-night is December 25–26, and twelfth-night is January 5–6.

These are the twelve days beginning on night of Christmas (December 25 ) and ending on the day of 6th January as Epiphany begins on (January 6 ). In the Middle Ages this period was one of continuous feasting and merrymaking, which climaxed on Twelfth Night  the traditional end of the Christmas season .     

The Twelve Days of Christmas is probably the most misunderstood part of the church year among Christians who are not part of liturgical church traditions. Contrary to much popular belief, these are not the twelve days before Christmas, but in most of the Western Church are the twelve days from Christmas until the beginning of Epiphany  (January 6th; the 12 days count from December 25th until January 5th). In some traditions, the first day of Christmas begins on the evening of December 25th but the following day is considered the First Day of Christmas