For countries like the United States, Canada, the United Kingdom, and most of Europe and Asia, winter occurs at the end and beginning of the calendar year.
Clean. Simple. December 1st is winter. February 28th is winter. March 1st is spring.
Because the Earth is tilted on its axis, the two halves of the planet experience opposite seasons at the same time. While those north of the equator are reaching for heavy coats in December, those south of the equator are enjoying the peak of summer.