From 1558 until 1829, Roman Catholics in England were not permitted to practice their faith openly. Someone during that era wrote this carol as a catechism song for young Catholics.

It has two levels of meaning the surface meaning plus a hidden meaning known only to members of their church. Each element in the carol has a code word for a religious reality which the children could remember.

The partridge in a pear tree was Jesus Christ.

Two turtle doves were the Old and New Testaments.

Three French hens stood for faith, hope and love.

The four calling birds were the four gospels of Matthew, Mark,  Luke & John.

The five golden rings recalled the Torah or Law, the first five books
of the Old Testament.

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-Love, Joy, Peace, Patience, Kindness, Goodness, Faithfulness, Gentleness, and Self Control.

The ten lords a-leaping were the ten commandments.

The eleven pipers piping stood for the eleven faithful disciples.

The twelve drummers drumming symbolized the twelve points of belief in the Apostles' Creed.
 

So there is your history lesson for today. Now you know how that strange song became a Christmas Carol.