寒假

It is specifically an academic term: students and teachers have a 寒假, and it centers on Chinese New Year (春节), so in mainland China it typically runs about a month across January and February. Two things matter for English speakers. First, 寒假 is not a general 'winter break' from work — office workers don't get a 寒假; that word belongs to schools. Second, the 假 here is read jià (fourth tone, 'holiday/leave'), not jiǎ (third tone, 'fake'), even though it's the same character.

Its paired opposite is 暑假 (shǔjià), the summer vacation. Register is neutral and everyday. Compare 假期 (jiàqī), the general word for any holiday period, of which 寒假 is one specific instance.