Each element in the carol has a code word for a religious reality which the children could remember. To fit the number scheme, when you reach number 9, representing the Fruits of the Holy Ghost, the originator combined 6 to make 3, taking the 6 fruits that were similar: the fruit in each parenthesis is the that was not named separately. There are actually Twelve Fruits of the Holy Ghost.
The "True Love" one hears in the song is not a smitten boy or girlfriend but Jesus Christ, because truly Love was born on Christmas Day. The partridge in the pear tree also represents Him because that bird is willing to sacrifice its life if necessary to protect its young by feigning injury to draw away predators.
According to Ann Ball in her book, HANDBOOK OF CATHOLIC SACRAMENTALS:
The two turtle doves were the Old and New Testaments
The three French hens stood for faith, hope, and love.
The four calling birds were the four gospels of Matthew, Mark, Luke, and John.
The five golden rings rerepresented the first five books of the Old Testament, which describe man's fall into sin and the great love of God in sending a Savior.
The six geese a-laying stood for the six days of creation.
Seven swans a-swimming represented the sevenfold gifts of the Holy Spirit-----Prophesy, Serving, Teaching, Exhortation, Contribution, Leadership, and Mercy.
The eight maids a-milking were the eight beatitudes.
Nine ladies dancing were the nine fruits of the Holy Spirit-----Charity, Joy, Peace, Patience [Forbearance], Goodness [Kindness], Mildness, Fidelity, Modesty, Continency [Chastity].
The ten lords a-leaping were the Ten Commandments.
The eleven pipers piping stood for the eleven faithful Apostles.
The twelve drummers drumming symbolized the twelve points of belief in The Apostles' Creed.
Original Source: Fr. Calvin Goodwin, FSSP, Nebraska
*http://www.catholicnewsagency.com/resources/advent/customs-and-traditions/the-history-of-the-twelve-days-of-christmas/
