Winter can be used in multiple ways. The way you're thinking of is the calendar based season which for the Northern hemisphere is Dec, Jan Feb (with each seaso

Winter can be used in multiple ways. The way you're thinking of is the calendar based season which for the Northern hemisphere is Dec, Jan Feb (with each season being three consecutive months) - more specifically this is called the meteorological season. The astronomical season (which is the more "official" one) starts on 22 December and ends on 21 March. But there's also a more informal usage, which is definitely prevalent in Europe, which is used to refer to the cold months of the year where you would typically have the heating on. For the UK this spans October to March for example. That doesn't mean you never have heating on in April, and it doesn't mean you _have_ to have the heating on in November. It's just a useful way of thinking about the period of time when gas/energy usage will be high and allows you to describe whether the period as a whole has been mild or particularly cold and the impact that's had on heating.