Definition of Twelve Days of Christmas:
(n) :
The season or time made up of the twelve days between Christmas and Epiphany; Christmas season, Christmas time, Christmastide, Twelvetide. The Christmas season traditionally ended with the feast of the Epiphany on January 6.
(n) :
An 18th-century English Christmas carol which describes a series of successively grand gifts given on each of the twelve days of Christmas.