"Time period of winter
Astronomically, it starts with the winter solstice (around December 21 in the Northern Hemisphere and June 21 in the Southern Hemisphere), and ends with the spring equinox (around March 21 in the Northern Hemisphere and September 21 in the Southern Hemisphere). In meteorology, it is by convention counted instead as the whole months of June, July and August in the Southern Hemisphere and December, January and February in the Northern Hemisphere.
However, in the United Kingdom and Ireland the winter solstice is traditionally considered as midwinter, the winter season beginning November 1 on All Hallows or Samhain. Winter ends and spring begins on Imbolc or Candlemas, which is February 1 or 2. 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 (May-July in the Southern).
In Chinese astronomy (and other East Asian calendars), winter is taken to commence on or around November 7, with the Jiéqì known as (立冬 lì dōng, literally "establishment of winter".)
In reality, the three-month period associated with the coldest average temperatures typically begins somewhere in late November or early December in the Northern Hemisphere. If "winter" is defined as the statistically coldest quarter of the year, then the astronomical definition is too late by almost all local climate standards, and the traditional English/Irish definition of November 1 (May 1 in the Southern Hemisphere) is almost always too early to fit this standard. No matter the recogning, winter is the only season that spans two calendar years in the northern hemisphere. (In other words, there are very few temperate climates in which the vernal equinox is on average colder than the winter solstice, and very few temperate climates in which Samhain is colder than Imbolc)"
Source(s):
http://en.wikipedia.org/wiki/Winter
