What do you mean by 150 before Tomb-Sweeping Day?

Tomb-Sweeping Day doesn't simply mean one day, but three days, called 150, Cold Food Festival, Tomb-Sweeping Day. It turns out that the "150" day from the solar term in beginning of winter to Tomb-Sweeping Day in the previous year is exactly 150, so people habitually call it 150. As the saying goes, "one hundred and fifty, swallowing soil" means that on the day of "one hundred and fifty", the swallow will fly from the south to the north, and then start to build a nest with mud.