'Winter' in calendar, solar terms in the Northern hemisphere , also called astronomical winter, starts on 21st December and ends on 20th March (there might be a one day shift now and then?). That is, the period from the winter solstice until the spring equinoxe. This is written on most calendars and that is what Google tells you when you look up 'winter'. I am flabbergasted that you never heard of that...

Now, usually the other definition people use relates to their locate climate for practical purposes.

Funnily enough we call that first day date “Midwinter Day” or “middle of the winter-day” here.

Maybe it it time to rename it to “Firstwinter Day”! ;)

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.

I find it very hard to believe you don’t know that winter starts on specific dates every year. Not really sure how grandparents fit into it either.

I have always known that winter according to social convention AND official calendars starts on December 1 every year. I have also known the scientific meteorological definition which is based on temperature.