W. K. Lis
Superstar
The start of winter is the solstice - which doesn't depend on the date. Winter then would start December 8 for those who haven't yet converted.
That was why we updated with the Gregorian calendar. The seasons were moving too far.
From link.
The pagan celebration of Saturn, the Roman god of agriculture and time, began as a single day, but by the late Republic (133-31 B.C.) it had expanded to a weeklong festival beginning December 17. (On the Julian calendar, which the Romans used at the time, the winter solstice fell on December 25.)