Haishi Day (or Cold Food Day) is the very day just before the Qingming Festival (also named Tomb Sweeping Festival, or Clear and Bright Festival). On the day every year, no fire or smoke is allowed and people shall eat cold food for the whole day.
According to the legend, the day is in memory of Jie Zhitui who lived in the Spring and Autumn Period (770-476BC).
Jie was a good official in the Jin State, working for Crown Prince Chong'er. When Jin State was in turmoil, Chong'er was forced to leave for other states with his henchmen, including Jie. On the way of exile they went through all kinds of hardships and difficulties. To save the starving Chong'er, Jie even cut the flesh off his own leg and boiled for Chong'er. After ascending the throne, Chong'er began to forget Jie by and by. Jie was so sad that he prepared to leave and live in seclusion with his mother in mountains.
Chong'er felt so guilty that he in person went to the mountains to look for Jie. For it was impossible to find him in the endless trees and hills, Chong'er ordered to set the mountain on fire, so as to force Jie out. But Jie didn't show up; he and his mother were found to be dead in arms after the fire was put out, together with a note written by him in blood: "I cut off my own flesh to dedicate to you, only to wish my king will always be clear and bright. "
In order to keep in memory of Jie Zitui, Chong'er issued an order to make the day Haishi Day, also named Cold Food Day. And on the Cold Food Day every year, no fire or smoke was allowed and people should eat cold food for the whole day.
It was not until the Qing Dynasty (1644-1911) about 300 years ago that the practice of Hanshi (or eating cold food) was replaced by that of Qingming, which had now become an important occasion for people to offer sacrifices to their ancestors.