Summer Solstice (Xiazhi) is the tenth of the twenty-four solar terms. It occurs when the Big Dipper points to the terrestrial branch wu and the sun reaches the ecliptic longitude of 90°, typically falling on June 20–22 in the Gregorian calendar. On the day of the Summer Solstice, the sun's rays strike directly at their northernmost point on the earth's surface, virtually perpendicular to the Tropic of Cancer. At this time, daylight hours across the Northern Hemisphere reach their annual maximum. For regions on and north of the Tropic of Cancer, the Summer Solstice is also the day of the year when the noon solar altitude is at its highest.

The Summer Solstice marks the turning point of the sun's northward journey. After the Summer Solstice, the point of direct solar incidence begins to move southward from the Tropic of Cancer, and daylight hours in the Northern Hemisphere gradually begin to shorten. For China's territories north of the Tropic of Cancer, the noon solar altitude progressively declines day by day after the Summer Solstice. For China's territories south of the Tropic of Cancer, the noon solar altitude begins to decline day by day only after the sun has returned southward and once again passed directly overhead following the solstice.

Xiazhi is not only one of the twenty-four solar terms, but was also, in ancient times, one of the festivals of the “Four Seasons and Eight Festivals” among the common people. Since ancient times, folk customs of worshipping deities and offering sacrifices to ancestors have been observed on the Summer Solstice. In addition, after the Summer Solstice, people commonly consume cooling, tonic broths (qing bu liang tang), herbal teas (liang cha), and sour plum drinks (suanmei tang) to escape the summer heat.