Yes that's a customary name for the winter solstice. Like Midsummer for the summer solstice.

As I wrote, in European tradition 'winter' starts much earlier than the winter solstice because quite obviously the cold and grey months start earlier.

I am still puzzled that you never heard of 21st December being the beginning of the (astronomical) winter, like 21st June is beginning of summer, etc.

Why would I ever have heard of it in the first place? It's not in use, not in social settings, not in official settings. Though I have learned it exists, and that some people _really_ like that definition. Reading this thread I am just as puzzled by you being puzzled, because I base my use of it on:

- Official communication (e.g. with tax authorities) use december first as first day of winter quarter.

- Social connotations use either a simplified version of the scientific/metrological definition (e.g. is it cold enough to be called winter yet? E.g. is it warm enough that winter has ended?) or calendar winter (dec-jan-feb).

- Historical reasons, late December is literally called "middle of the winter". Same thing with summer, it officially begins on June 1st and the summer solstice marks the "middle of summer" the same way "Mittwoch" marks the middle of the week.

- In the scientific literature I occasionally read (e.g. meteorology and biology) I read the definition is when the average daily temperature determines winter and never based on astronomy.