The world is divided into two hemispheres: the Northern and Southern Hemispheres. In the Northern Hemisphere ─ where countries include the entire continent of North America (and Central America) and all of Europe, most of Asia, two thirds of Africa, and about 10% of South America ─ the first official day of winter in 2017 is December 21st.
The first day of winter is not actually measured as a “day” but a moment, when the Sun is exactly overhead the Tropic of Capricorn. This day, which is the start of the December Solstice or the winter solstice, is the shortest day of the year.
Conversely, in the Southern Hemisphere, it will be the summer solstice and the longest day of the year. Not everyone experiences winter at the same time. (Info sourced from timeanddate.com, pic from pinterest)