What does it mean to call sweeping graves in rural areas of Hebei a new grave with three cold foods and four sunny days?

Take Tomb-Sweeping Day as Tomb-Sweeping Day, Tomb-Sweeping Day is a cold food the day before, Tomb-Sweeping Day is an old grave two days ago, and Tomb-Sweeping Day is a new grave three days ago.

Take Tomb-Sweeping Day on April 4th as an example, April 1 is the heart and lung, April 2nd is the old grave, April 3rd is the cold food, and April 4th is the Tomb-Sweeping Day.

Between last year's Tomb-Sweeping Day and this year's Tomb-Sweeping Day, relatives died, which was a new grave. In the first three years, between Tomb-Sweeping Day, it was an old grave, and the rest went to Tomb-Sweeping Day.