When are the 12 days of Christmas? (text from, and full article at: http://www.cresourcei.org/cy12days.html)
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 with the following day considered the First Day of Christmas (December 26th). In these traditions, the twelve days begin December 26 and include Epiphany on January 6.
And the song?...
The popular song "The Twelve Days of Christmas" is usually seen as simply a nonsense song for children. However, some have suggested that it is a song of Christian instruction dating to the 16th century religious wars in England, with hidden references to the basic teachings of the Faith. They contend that it was a mnemonic device to teach the catechism to youngsters. The "true love" mentioned in the song is not an earthly suitor, but refers to God Himself. The "me" who receives the presents refers to every baptized person who is part of the Christian Faith. Each of the "days" represents some aspect of the Christian Faith that was important for children to learn.