The Met Office classify the seasons as: Spring: March, April and May. Summer: June, July and August. Autumn: September, October and November. Winter: December, January and February.
By that reasoning Winters starts tomorrow (December 1st.)
They do this in the main for statistical reasons rather than the old way of thinking Winter starts on the Winter Solstice (usually around December 22nd.) Likewise Summer used to start on the Summer Solstice (usually around June 21st.)
Of course nobody has told the weather about this, so it just does what it does.
