Today is Jungbok, the second of the Sambok, the Three Hot Days (i.e. Dog Days of Summer).
The Sambok have long been described as the hottest days of the year. The concept was disseminated into Korean lore from China, where the Sambok were calculated according to the Ten Celestial Stems (cheon-gan/ 천간). This system was used in counting and astrological calculations, including counting the days of the week. In the system, therefore, a week consisted of ten days: gap, eul, pyeong, cheong, mu, gi, gyeong, haeng, im, and gye. The Sambok are calculated according to this system. The first Sambok, called Chobok, falls on the third gyeong day after summer solstice (haji/하지). Jungbok is ten days after that.
People often observed bok-jae (복제) on Sambok, a rite for giving offerings to ancestral spirits and gods related to agriculture. As was common on most special days, the weather on Sambok was considered to foretell coming harvests.
Traditionally, dog or chicken soups were eaten on the Sambok. Bok-nori (복놀이) was the custom of preparing and enjoying these foods in the shade, often in cooler areas around mountains and valleys. These food traditions still exist today, the lore still suggesting these foods help one resist the searing summer heat. Interestingly, eating muk-namul (aged/dried vegetables) on the First Full Moon (Daeboreum/대보름; in mid-winter) was said to help one prepare for the summer heat.