Between the years 1558‑1829, English Catholics were not permitted to practice their faith openly. Without regular Mass, sacraments, or catechism lessons from the priest, parents sought help to assist their children in learning and remembering their Faith. This song was created to keep the Catholic faith alive, though hidden for the time. The 12 days are from the feast of Christmas, December 25th, to the feast of Epiphany, January 6th.
Instead of referring to a suitor, the “true love” refers to God Himself. The “me” who receives the presents symbolizes every baptized person. The “partridge in a pear tree” is Jesus Christ. A mother partridge will feign injury to decoy predators from her helpless nestlings. The children hearing this song would have known that, and therefore would have understood the parallel between the acts of the mother bird and the sacrifice of Christ.
The other items continue the symbolism:
2 turtle doves the Old and New Testaments;
3 French hens Faith, Hope and Charity;
4 calling birds the Four Gospels;
5 golden rings the first five books of the Old Testament
6 geese a laying the days of creation;
7 swans a swimming the seven gifts of the Holy Spirit;
8 maids a milking the eight Beatitudes;
9 ladies dancing nine choirs of angels;
10 lords a leaping the Ten Commandments;
11 pipers piping the eleven faithful Apostles;
12 drummers drumming the twelve points of belief in the Apostles' Creed.