On the last day of every lunar year, an announcing cult, cung tien thuong, is performed to invite the dead forefathers to return home to celebrate Tet holidays with their families. During the last days before Tet, all family members visit their ancestors’ graves; they clean and decorate the graves, in the same manner that the livings clean and decorate their houses to welcome the New Year.
On the anniversary of an ancestor’s death, descendants and relatives unite and prepare a feast to worship the dead people and to ask for health and happiness for themselves. From generation to generation, ancestor worshipping customs have been religiously preserved. There are some small variations between those customs among the many Vietnamese ethnic groups, but the common theme of fidelity and gratitude towards the ancestors remains.
Source vietnamtourism.com