“On the coldest day you appreciate the warmth of the fire
and the blanket around you the most .
On the darkest night you can see the stars of the northern sky
and endlessness of the universe the clearest.”

The Making of Winter Poem – Secret Garden

In Celtic nations such as Ireland, the winter solstice is traditionally considered as midwinter.

Thus, the winter season beginning 1 November, on All Hallows, or Samhain and ends as spring begins on Imbolc, or Candlemas, which is 1 or 2 February.

This system of seasons is based on the length of days exclusively. (The three-month period of the shortest days and weakest solar radiation occurs during November, December and January in the Northern Hemisphere.

In Chinese astronomy and other East Asian calendars, winter is taken to commence on or around 7 November, with the Jiéqì (known as 立冬 lì dōng – literally, “establishment of winter”).

Many mainland European countries tended to recognize Martinmas or St. Martin’s Day (11 November), as the first calendar day of winter. The day falls at the midpoint between the old Julian equinox and solstice dates. Also, Valentine’s Day (14 February) is recognized by some countries as heralding the first rites of spring, such as flowers blooming.

Seasonal lag means that the coldest period does normally follows the solstice by a few weeks. The beginning and end of the meteorological winter is therefore usually considered to span the entirety of December, January, and February in the Northern Hemisphere. As they three-month period associated with the coldest average temperatures typically begins somewhere in late November or early December in the Northern Hemisphere and lasts through late February or early March.
Astronomically, the winter solstice on 21st December, is considered the start of winter

This “thermological winter” is later than the daylight (Celtic) definition but earlier than the solstice delimited definition, which defines the period of winter based on astronomical fixed points (i.e. based solely on the position of the Earth in its orbit around the Sun), regardless of weather conditions. Thus winter is considered to beginat the winter solstice 21st December and end at the 29th March spring equinox.

Since by almost all definitions valid for the Northern Hemisphere, winter spans 31 December and 1 January, the season is split across years, Each calendar year includes parts of two winters. This causes ambiguity in associating a winter with a particular year.

Ecological reckoning and activity

Ecological reckoning of winter differs from calendar-based by avoiding the use of fixed dates.
It is one of six seasons recognized by most ecologists who customarily use the term hibernal for this period of the year. The hibernal season coincides with the main period of biological dormancy each year whose dates vary according to local and regional climates in temperate zones of the Earth.

The other ecological seasons being prevernal, vernal, estival, serotinal, and autumnal. The appearance of flowering plants like the crocus can mark the change from ecological winter to the prevernal season as early as late January in mild temperate climates.

https://en.wikipedia.org/wiki/Winter