Xiazhi 夏至 — Summer Solstice

The longest day — the sun's northernmost point and the peak of yang. Xiazhi falls around 21 June. It is one of the four cardinal terms and the point of maximum daylight in the northern hemisphere.

It is a solar term — a date fixed by the sun's position, not by the moon. The longest day — the sun's northernmost point and the peak of yang. The solstice/equinox framing is the oldest layer of the calendar: the astronomical event is measured, and the cosmological reading is attached to it afterwards rather than the reverse.

In the yin-yang scheme this is the moment of fullest yang, and the classical observation is that the turn begins here rather than at the height of the heat — yang peaks and yin begins in the same instant. It is grouped under Fire (火) in the five-element system.