From the web - and I always thought this...
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).