The 12 days of Christmas start on Christmas and end on January 5, the eve of the Feast of Epiphany.

12-day advent calendars are a fairly recent invention that mirrors the 12-days of Christmas, but has no direct correspondence to anything in any traditional Christian religious calendar (the more common 24-day format is also a modern, but less recent, invention detached from the religious calendar, that simplifies by ignoring the floating start date of advent and always starting on Dec. 1.)