“Ullambana” means “To liberate up-side-down”. In other words, to liberate the sentient beings who suffer in the wretched realms. by relying on the merits of offerings to the venerable sangha everywhere.
Its origin was from Buddha’s teaching requested by Maudgalyayana. When Venerable Maudgalyayana had just attained the six supernatural powers, he wished to save his parents in order to repay their kindness in raising him. Using his Dharma eye to search the world, he saw that his deceased mother had been reborn in the realm of hungry ghosts. She could not find any food or drink, and she was all skin and bones. Maudgalyayana was saddened. He immediately filled an alms bowl with food and offered it to his mother. But before the food could reach her mouth, it turned in to burning charcoal and she was not able to take it. Maudgalyayana cried out with grief. He rushed back to tell the Buddha what had happened.
The Buddha said, “The roots of your mother’s transgressions are deep and entangled. Even though you are pious and your cries move heaven and earth, your powers alone are not sufficient to save her. Heavenly beings, spirits of the ground, demons, heretics, and the Four Heavenly Kings cannot help her either. You need the august spiritual powers of the sangha assembly of the ten directions to free her. I will now instruct you on the way of salvation so that all those in the same plight will be freed from grief and suffering.” Thus, the Buddha gave the teaching on how to make offerings on behalf of the parents to the sangha. This, along with the day of confessions for sangha of the ten directions should be done on the fifteenth day of the seventh lunar month which is the end of annual summer retreat. In “The Ullambana Sutra”, the Buddha elaborated on the ways to make offerings and the reasons for these offerings.