The "partridge in a pear tree" is Jesus Christ.
The three French hens stand for faith, hope and love.
The four calling birds are the four Gospels.
The five gold rings recall the torah (Law) the first five books of the Old Testament.
The six geese a-laying stand for the six days of creation.
The seven swans a-swimming represent the sevenfold gifts of the Spirit.
The eight maids a-milking are the eight beatitudes.
The nine ladies dancing are the nine fruit of the Spirit (Gal.5).
The ten lords a-leaping are the Ten Commandments.
The eleven pipers piping stand for the eleven faithful disciples.
The twelve drummers drumming symbolize the 12 points of belief in the Apostles Creed.
There you have it... the hidden meaning of "The Twelve Days of Christmas" and the secret behind the song.