"Ullambana is the festival of deliverance, and advocates and reinforces the concept of filial piety. The word ullambana translates into "deliverance from suffering", and specifically refers to the salvation that is granted to tormented souls in hell." 
(Source: chuadieuphap.us)

Ullambana is a festival of Buddhist origins. It is celebrated during six continuous evenings and consists mainly of the chanting of Amitabha Sutra. On the seventh day, a Filial Piety Dharma Assembly is held with a Lamp Lighting Ceremony.

The Filial Piety Dharma Assembly develops gratitude for parents, ancestors, all sentient beings, who could have been parents in past lives. Also, transferring merits to all the departed and all sentient beings in all the Buddhist realms.

Lamp Lighting: the light symbolizes knowledge and the dispelling of darkness. Darkness represents ignorance. Therefore, lighting a lamp symbolizes the destruction of all negative forces.

Many celebrations of the dead combine similar beliefs and actions. The lighting of lamps or lanterns is one that you can find in other religions, there is the lighting of lamps and lanterns during Diwali/Deepavali and the Mid-Autumn festival, for example...