The following is an URBAN LEGEND (see http://www.snopes.com/holidays/christmas/12days.asp).

"From 1558 until 1829 Roman Catholics in England were not allowed to
practice their faith openly. During that era someone wrote 'The
Twelve Days of Christmas' as a kind of secret catechism that could
be sung in public without the risk of persecution. The song has two
levels of interpretation:  the surface meaning plus a hidden meaning
known only to members of the church.  Each element in the carol is a
code word for a religious reality."
 
    That is, ...
   *The partridge in a pear tree is Jesus Christ. 
   *The two turtledoves are the Old and New Testaments, respectively.
   *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. 
   *Seven swans a-swimming represent the sevenfold gifts of the 
       Spirit. 
   *The eight maids a-milking are the eight beatitudes. 
   *Nine ladies dancing? These are the nine fruits of the Spirit 
       (Gal. 5). 
   *The ten lords a-leaping are the Ten Commandments. 
   *Eleven pipers piping stand for the eleven faithful disciples. 
   *Twelve drummers drumming symbolize the 12 points of belief in 
       the Apostles' Creed.