Maudgalyayana and filial gratitude
Maudgalyayana (Mục Kiền Liên) was one of the Buddha's great disciples, known for his spiritual powers. After years of practice he remembered his late mother and wished to know where she was.
According to the Vu Lan sutra he saw his mother suffering from hunger. He offered her rice, but before it reached her mouth it turned to flame. Sorrowful, he asked the Buddha.
The Buddha taught that one person's strength was not enough. On the full-moon day of the seventh lunar month, when the monastics end the three-month rains retreat, offer to them with a sincere heart. The shared merit of that community would turn toward parents and ancestors.
From this story Vu Lan became the Vietnamese season of filial gratitude. Many people visit a temple, see their parents and remember those who have passed.
Traditions tell this story somewhat differently, so this is only a short retelling.