Although today's society has developed more modern than the time of our grandparents, but the custom of viewing good and bad days according to age or choosing a wedding date, a good day to break ground, consider a beautiful day to open, buy a car, buy a house... It can be said that the important things are carefully selected. Seeing good and bad days before doing big things has now become a beautiful traditional culture of Vietnamese people. However, what is a good day, what is a good day, what