I would like to ask how to calculate the time before burning seven, three seven and five seven?

Burning Seven is a sacrifice day every seven days from the date of death of the deceased, which is called the first seven days, the last seven days, the third seven days, the fourth seven days, the fifth seven days, the sixth seven days and the last seven days.

"The first seven" means "the deceased goes home to say goodbye to their families on this day!" , but this is also a folk feudal superstition; "The first seven days" is one of the folk funeral customs in China, which means that the "soul" of the "deceased" will return home on this day, and then a "farewell ceremony" will be held to allow him to be reincarnated.

Extended data:

In China's folk custom, the first seven is to "welcome the dead home". On this day, we should cook a sumptuous dish for their souls to enjoy, that is, as the saying goes, they will be reborn after eating it, and burn more for the "dead" before leaving this world completely.

There is a folk saying that people have three souls and seven spirits. Here, "burning seven" means July 49, and "Yan" is interrogated every seven days, and then "one spirit is exhausted" until "seven spirits are exhausted". Therefore, every seven days, it is necessary to "burn Mingbi" to plead with "Yan", so there are the seventh day, the second day and the third day.

Baidu Encyclopedia-Top Seven