Incorrect! You may be thinking of the Catholic period of Christmas which begins on Christmas Eve and typically lasts for 20 days. No, the Twelve Days of Christmas harkens back to a much more ancient Celtic tradition.
Back in pagean times in neolithic Britain (circa 5,500 BCE), the keepers of the giga sundial known to us as
Stonehenge were aware that for a 12 day period, the time of sunset remained the same - hence
solstice, or '
sun-stand-still'. This period typically starts around Dec 9th or 10th (on the Gregorian Calendar) and ends a day or two before the solstice. Here are Sun times at Salisbury Plain for the month of Dec 2023:
Calculations of sunrise and sunset in Salisbury Plain – England – United Kingdom for April 2024. Generic astronomy calculator to calculate times for sunrise, sunset, moonrise, moonset for many cities, with daylight saving time and time zones taken in account.
www.timeanddate.com
This was a great time of gathering and feasting on Salisbury Plain, when the shaman's had their time with the common folk (kinda like Wall-E today). So this grand old tradition is not something that Brits were willing to give up for a relatively late addition to their calendar: Christmas. (the actual birth day is another story, but very OT)
Cheers!