what's the deal with 12-days advent calendars lately?
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.)
Advent begins on the fourth Sunday before Christmas, which was Nov 30 this year. It ends on Dec 24. Therefore it is technically anywhere from 22 to 28 days long.
Advent calendars begin on Dec 1 and end on Dec 25.