Bok-nal means it's the most hottest days in a year. Also, Koreans call it Sam-bok-deow-we. It's between June and July, and there's 3 Bok-nals. First one is called Cho-bok (7/13 in 2013), second one is called Jung-bok (7/23 in 2013), and the last one is called Mal-bok (8/12 in 2013). And each day, people tend to eat some food which could make them invigorated. Which means for self-preservation, Koreans eat special foods which could make people get stronger enough to spend summer season well. Most famous special foods are dog-meat soup and Samgyetang (chicken soup with ginseng). Koreans think that eating hot foods even in the hottest days means fight fire with fire. And believed to make body stronger by that. So people usually eat hot healthy food in these days, not cold foods.
Dog-meat soup is believed to restores energy to tired body. Following to the eastern medicinal manual, it says dog-meat makes the five viscera relaxed, controls the blood vessel, makes stomach and intestine stronger and makes waist and knee warm so that it improves person's energy. Koreans believed that don-meat soup is such a healthy food, so in Chosun dynasty, people even already knew how to cook dogs well.
But in some areas, people believed that eating don-meat soup brings bad luck, some Koreans banned to eat don-meat soup. Also, in history, some religion banned to eat it. These days, not a lot of people are eating dog-meat soup, but still there's some people who eat it. Because of these reasons, as a substitute for dog-meat soup, some people enjoyed Samggyetang (chicken soup with ginseng).
Samggyetang is made of young chicken and put ginseng, jujube, glutinous rice inside of it and boil it for long time. Sometimes, people use black chicken instead of young chicken. Eating chicken keeps people from getting cold and it's good for the stamina. Cure-all food like Ginseng and garlics are added in this, Samggyetang is believed to be a perfect health food.
And some people also eat Patjuk as a health food. Patjuk is believed to prevent people to get affected by heat and diseases. Also, in hot weather, people thought that Patjuk can expel ghost so lots of people enjoyed it.