First day of winter
The next 3 months with the December Solstice at their centre represent that period when Northern hemisphere locations begin to slide into the darkest and coldest phase, while sometimes the period preceding or following the 3 months may be just as cold, the underlying daylight/darkness asymmetries remain the same regardless of location.
The justification for astronomical perspectives is certainly the Polar day/night cycle with sunrise/sunset on the Equinoxes and midnight/noon on the Solstices so 6 weeks either side of Polar midnight this December is roughly November 1st and March 1st.
Much like daily noon, the warmest temperatures are not at noon (Solstices) but the following hours up to 3 PM so in terms of the Polar day/night cycle that would place the warmest temperatures around February/March in the Southern hemisphere and January/February as the coldest in the Northern.
All things are easily understood in terms of the Polar day/night cycle even without its surface rotational cause (parallel to the orbital plane) so instead of 24 hours, this cycle uses 12 months.